Semiconductor device

ABSTRACT

To provide a semiconductor device including a capacitor whose charge capacity is increased without reducing the aperture ratio. The semiconductor device includes a transistor including a light-transmitting semiconductor film, a capacitor where a dielectric film is provided between a pair of electrodes, an insulating film provided over the light-transmitting semiconductor film, and a light-transmitting conductive film provided over the insulating film. In the capacitor, a metal oxide film containing at least indium (In) or zinc (Zn) and formed on the same surface as the light-transmitting semiconductor film in the transistor serves as one electrode, the light-transmitting conductive film serves as the other electrode, and the insulating film provided over the light-transmitting semiconductor film serves as the dielectric film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/957,819, filed Aug. 2, 2013, now allowed, which claims the benefit offoreign priority applications filed in Japan as Serial No. 2012-173349on Aug. 3, 2012, Serial No. 2012-178941 on Aug. 10, 2012 and Serial No.2012-188093 on Aug. 28, 2012, all of which are incorporated byreference.

TECHNICAL FIELD

The invention disclosed in this specification and the like relates to asemiconductor device.

BACKGROUND ART

In recent years, flat panel displays such as liquid crystal displays(LCDs) have been widespread. In each of pixels provided in the rowdirection and the column direction in a display device such as a flatpanel display, a transistor serving as a switching element, a liquidcrystal element electrically connected to the transistor, and acapacitor connected to the liquid crystal element in parallel areprovided.

As a semiconductor material of a semiconductor film of the transistor, asilicon semiconductor such as amorphous silicon or polysilicon(polycrystalline silicon) is generally used.

Metal oxides having semiconductor characteristics (hereinafter referredto as oxide semiconductors) can be used for semiconductor films intransistors. For example, techniques for forming transistors using zincoxide or an In—Ga—Zn-based oxide semiconductor are disclosed (see PatentDocuments 1 and 2).

REFERENCES

[Patent Document 1] Japanese Published Patent Application No.2007-123861

[Patent Document 2] Japanese Published Patent Application No.2007-096055

DISCLOSURE OF INVENTION

In a capacitor, a dielectric film is provided between a pair ofelectrodes at least one of which is formed, in many cases, using alight-blocking film partly serving as a gate electrode, a sourceelectrode, a drain electrode, or the like of a transistor.

As the capacitance value of a capacitor is increased, a period in whichthe alignment of liquid crystal molecules of a liquid crystal elementcan be kept constant in the state where an electric field is applied canbe made longer. When the period can be made longer in a display devicewhich displays a still image, the number of times of rewriting imagedata can be reduced, leading to a reduction in power consumption.

One of methods for increasing the charge capacity of a capacitor is toincrease the area occupied by the capacitor, specifically, to increasethe area of a portion where a pair of electrodes overlap each other.However, when the area of a light-blocking conductive film is increasedto increase the area of a portion where a pair of electrodes overlapswith each other, the aperture ratio of a pixel is lowered and thusdisplay quality of an image is degraded.

In view of the above problems, it is an object of one embodiment of thepresent invention to provide a semiconductor device including acapacitor with increased charge capacity and having a high apertureratio.

One embodiment of the present invention is a semiconductor deviceincluding a transistor and a light-transmitting capacitor. Specifically,in the capacitor in the semiconductor device, a light-transmittingsemiconductor film serves as one electrode of the capacitor, alight-transmitting conductive film serves as the other electrode of thecapacitor, and a light-transmitting insulating film serves as adielectric film.

One embodiment of the present invention is a semiconductor deviceincluding a transistor including a light-transmitting semiconductorfilm, a capacitor where a dielectric film is provided between a pair ofelectrodes, an insulating film provided over the light-transmittingsemiconductor film, and a light-transmitting conductive film providedover the insulating film. In the capacitor, a light-transmittingsemiconductor film formed on the same surface as the light-transmittingsemiconductor film in the transistor serves as one electrode, thelight-transmitting conductive film serves as the other electrode, andthe insulating film provided over the light-transmitting semiconductorfilm serves as the dielectric film.

The light-transmitting semiconductor film can be formed using an oxidesemiconductor. This is because an oxide semiconductor has an energy gapas wide as 3.0 eV or more and high visible-light transmissivity.

In the case where a semiconductor film formed in a step of forming thesemiconductor film included in the transistor is used as one electrodeof the capacitor, the conductivity of the semiconductor film may beincreased. For example, it is preferable to add one or more selectedfrom boron, nitrogen, fluorine, aluminum, phosphorus, arsenic, indium,tin, antimony, and a rare gas element to the semiconductor film. An ionimplantation method, an ion doping method, or the like may be employedto add the element to the semiconductor film. Alternatively, thesemiconductor film may be exposed to plasma containing the element toadd the element. In that case, the conductivity of the semiconductorfilm serving as one electrode of the capacitor is greater than or equalto 10 S/cm and less than or equal to 1000 S/cm, preferably greater thanor equal to 100 S/cm and less than or equal to 1000 S/cm.

With the above structure, the capacitor transmits light and thus can beformed large (in a large area) in a pixel region except a portion wheretransistors are formed in the pixel. For this reason, the semiconductordevice can have charge capacity increased while improving the apertureratio. Accordingly, the semiconductor device can have excellent displayquality.

In the capacitor, the insulating film provided over the semiconductorfilm included in the transistor is used as the dielectric film;therefore, the dielectric film can have the same layered structure asthe insulating film. For example, in the case where the insulating filmprovided over the semiconductor film included in the transistor has alayered structure of an oxide insulating film and a nitride insulatingfilm, the dielectric film of the capacitor can have a layered structureof the oxide insulating film and the nitride insulating film.

In the case where in the capacitor, the insulating film provided overthe semiconductor film included in the transistor has a layeredstructure of an oxide insulating film and a nitride insulating film,only a portion of the oxide insulating film in a region where thecapacitor is formed is removed after the oxide insulating film isformed, whereby the dielectric film of the capacitor can have asingle-layer structure of the nitride insulating film. In other words,the nitride insulating film is in contact with an oxide semiconductorfilm serving as the pair of electrodes of the capacitor, whereby defectstates (interface states) at the interface between the nitrideinsulating film and the oxide semiconductor film or nitrogen containedin the nitride insulating film diffuses into the oxide semiconductorfilm, leading to an increase in the conductivity of the oxidesemiconductor film. Further, the thickness of the dielectric film can bereduced; therefore, an increase in the charge capacity of the capacitorcan be achieved.

When the nitride insulating film is in contact with the semiconductorfilm in the capacitor as described above, a step of adding an elementwhich increases the conductivity to the semiconductor film by an ionimplantation method, an ion doping method, or the like can be skipped;therefore, the yield of the semiconductor device can be increased andthe manufacturing cost thereof can be reduced.

In the case where the semiconductor film included in the transistor isan oxide semiconductor film and the insulating film over thesemiconductor film is a stack of an oxide insulating film and a nitrideinsulating film, the oxide insulating film is preferably less likely totransmit nitrogen, that is, the oxide insulating film preferably has abarrier property against nitrogen.

With the above structure, one of or both nitrogen and hydrogen can beprevented from diffusing into the oxide semiconductor film as thesemiconductor film included in the transistor, so that variations in theelectrical characteristics of the transistor can be suppressed.

In the above, an organic insulating film may be provided between thelight-transmitting conductive film and the insulating film provided overthe semiconductor film included in the transistor. With such astructure, parasitic capacitance between the light-transmittingconductive film and a conductive film partly serving as a sourceelectrode, a drain electrode, or the like can be reduced, so thatfavorable electrical characteristics of the semiconductor device can beachieved. For example, signal delays of the semiconductor device can bereduced.

To increase the charge capacity of the capacitor, it is effective toreduce the thickness of the dielectric film; therefore, it is preferableto remove a portion of the organic insulating film which is over aregion where the capacitor is formed. In the case where thesemiconductor film included in the transistor is an oxide semiconductorfilm, to prevent hydrogen, water, and the like contained in the organicinsulating film from diffusing into the oxide semiconductor film, it ispreferable to remove a portion of the organic insulating film whichoverlaps with the semiconductor film included in the transistor.

In the case where the light-transmitting conductive film is connected tothe transistor, the light-transmitting conductive film serves as a pixelelectrode.

In the case where the light-transmitting conductive film serves as apixel electrode, a capacitor line extends in the direction parallel witha scan line, on the same surface as the scan line. One electrode(semiconductor film) of the capacitor is electrically connected to thecapacitor line through a conductive film formed at the same time asformation of source and drain electrodes of the transistor.

The capacitor line does not necessarily extend in the direction parallelwith a scan line, on the same surface as the scan line. The capacitorline may extend in the direction parallel with a scan line including thesource electrode or the drain electrode of the transistor, on the samesurface as the signal line, and may be electrically connected to oneelectrode (the semiconductor film one) of the capacitor.

The capacitor line may be formed using the semiconductor film includedin the capacitor.

The capacitor line may be connected to each of capacitors included in aplurality of adjacent pixels. In this case, the capacitor line may beprovided between the adjacent pixels.

In the case where the conductivity of the semiconductor film included inthe capacitor is high, the semiconductor film included in the capacitormay be connected to the transistor. In this case, the semiconductor filmincluded in the capacitor serves as a pixel electrode, and thelight-transmitting conductive film serves as a common electrode and thecapacitor line.

In the case where the semiconductor film formed in the step of formingthe semiconductor film included in the transistor serves as oneelectrode of the capacitor, the conductive film in contact with thesemiconductor film and the capacitor line may be provided in contactwith an end portion of the semiconductor film and, for example, can beprovided in contact with the semiconductor film along the outerperiphery thereof. With such a structure, the contact resistance betweenthe semiconductor film and the conductive film is reduced.

The light-transmitting capacitor can be formed using a formation processof the transistor. One electrode of the capacitor can be formed using aformation process of the semiconductor film included in the transistor.The dielectric film of the capacitor can be formed using a formationprocess of the insulating film provided over the semiconductor filmincluded in the transistor. The other electrode of the capacitor can beformed using a formation process of the light-transmitting conductivefilm serving as a pixel electrode or a common electrode. Thus, thesemiconductor film included in the transistor and one electrode of thecapacitor are formed using the same metal element.

A fabrication method of a semiconductor device of one embodiment of thepresent invention is one embodiment of the present invention.

According to one embodiment of the present invention, a semiconductordevice including a capacitor whose charge capacity is increased whileimproving the aperture ratio can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1A illustrates a semiconductor device of one embodiment of thepresent invention and FIGS. 1B and 1C are circuit diagrams eachillustrating a pixel;

FIG. 2 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIGS. 4A and 4B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device of one embodiment of the presentinvention;

FIGS. 5A and 5B are cross-sectional views illustrating the manufacturingmethod of a semiconductor device of one embodiment of the presentinvention;

FIG. 6 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 7 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 8 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 9 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIGS. 10A and 10B are cross-sectional views illustrating thesemiconductor device of one embodiment of the present invention;

FIG. 11 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 12 is a cross-sectional view illustrating the semiconductor deviceof one embodiment of the present invention;

FIG. 13 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 14 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 15 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 16 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 17 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 18 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 19 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 20 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 21 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIGS. 22A and 22B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device of one embodiment of the presentinvention;

FIGS. 23A and 23B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device of one embodiment of the presentinvention;

FIG. 24 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 25 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 26 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIGS. 27A and 27B are cross-sectional views illustrating a manufacturingmethod of a semiconductor device of one embodiment of the presentinvention;

FIGS. 28A and 28B are cross-sectional views illustrating themanufacturing method of a semiconductor device of one embodiment of thepresent invention;

FIG. 29 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIGS. 30A to 30C are top views each illustrating a semiconductor deviceof one embodiment of the present invention;

FIGS. 31A and 31B are cross-sectional views each illustrating asemiconductor device of one embodiment of the present invention;

FIGS. 32A and 32B are a cross-sectional view and a top view illustratinga semiconductor device of one embodiment of the present invention, andFIG. 32C is a cross-sectional view illustrating a semiconductor deviceof one embodiment of the present invention;

FIGS. 33A to 33C illustrate electronic devices in each of which asemiconductor device of one embodiment of the present invention is used;

FIG. 34A to 34C illustrate an electronic device in which a semiconductordevice of one embodiment of the present invention is used;

FIG. 35 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIGS. 36A and 36B are cross-sectional view each illustrating asemiconductor device of one embodiment of the present invention;

FIG. 37 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 38 is a graph showing a capacitor included in a semiconductordevice of one embodiment of the present invention;

FIGS. 39A and 39B each illustrate an operating method of a capacitorincluded in a semiconductor device of one embodiment of the presentinvention;

FIGS. 40A and 40B are top views illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 41 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention;

FIG. 42 is a cross-sectional view illustrating the structure of atransistor used for calculation;

FIGS. 43A and 43B are cross-sectional views each illustratingequipotential curves of a transistor which are obtained by calculation;

FIGS. 44A and 44B are graphs each showing current-voltage curves of atransistor which are obtained by calculation;

FIG. 45 illustrates a display image of a liquid crystal display device;

FIG. 46 is a top view illustrating a semiconductor device of oneembodiment of the present invention;

FIG. 47 is a cross-sectional view illustrating a semiconductor device ofone embodiment of the present invention; and

FIG. 48 is a top view illustrating a semiconductor device of oneembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description below,and it is easily understood by those skilled in the art that modes anddetails disclosed herein can be modified in various ways. In addition,the present invention is not construed as being limited to the followingdescriptions of the embodiments and example.

Note that in structures of the present invention described below, thesame portions or portions having similar functions are denoted by commonreference numerals in different drawings, and descriptions thereof arenot repeated. Further, the same hatching pattern is applied to portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

Note that in each drawing described in this specification, the size, thefilm thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, the embodiments and example of thepresent invention are not limited to such scales in the drawings.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. In addition,the ordinal numbers in this specification and the like do not denoteparticular names which specify the present invention.

Functions of a “source” and a “drain” in the present invention aresometimes replaced with each other when the direction of current flow ischanged in circuit operation, for example. Therefore, the terms “source”and “drain” can be interchanged with each other in this specification.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or a voltage, and a potential and a voltage are usedas synonymous words in many cases. Thus, in this specification, apotential may be rephrased as a voltage and a voltage may be rephrasedas a potential unless otherwise specified.

In this specification, in the case where etching treatment is performedafter photolithography treatment, a mask formed in the photolithographytreatment is removed after the etching treatment.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention will be described with reference to drawings. Notethat in this embodiment, a semiconductor device of one embodiment of thepresent invention will be described taking a liquid crystal displaydevice as an example.

<Structure of Semiconductor Device>

FIG. 1A illustrates an example of a semiconductor device. Thesemiconductor device in FIG. 1A includes a pixel portion 100, a scanline driver circuit 104, a signal line driver circuit 106, m scan lines107 which are arranged in parallel or substantially in parallel andwhose potentials are controlled by the scan line driver circuit 104, andn signal lines 109 which are arranged in parallel or substantially inparallel and whose potentials are controlled by the signal line drivercircuit 106. Further, the pixel portion 100 includes a plurality ofpixels 101 arranged in a matrix. Furthermore, capacitor lines 115arranged in parallel or substantially in parallel are provided along thescan lines 107. Note that the capacitor lines 115 may be arranged inparallel or substantially in parallel along the signal lines 109.

Each scan line 107 is electrically connected to the n pixels 101 in thecorresponding row among the pixels 101 arranged in m rows and n columnsin the pixel portion 100. Each signal line 109 is electrically connectedto the m pixels 101 in the corresponding column among the pixels 101arranged in m rows and n columns Note that m and 11 are each an integerof 1 or more. Each capacitor line 115 is electrically connected to the npixels 101 in the corresponding row among the pixels 101 arranged in mrows and n columns Note that in the case where the capacitor lines 115are arranged in parallel or substantially in parallel along the signallines 109, each capacitor line 115 is electrically connected to the mpixels 101 in the corresponding column among the pixels 101 arranged inm rows and n columns.

FIG. 1B is an example of a circuit diagram of the pixel 101 included inthe semiconductor device illustrated in FIG. 1A. The pixel 101 in FIG.1B includes a transistor 103 which is electrically connected to the scanline 107 and the signal line 109, a capacitor 105 one electrode of whichis electrically connected to a drain electrode of the transistor 103 andthe other electrode of which is electrically connected to the capacitorline 115 which supplies a constant potential, and a liquid crystalelement 108. A pixel electrode of the liquid crystal element 108 iselectrically connected to the drain electrode of the transistor 103 andthe one electrode of the capacitor 105, and an electrode (counterelectrode) facing the pixel electrode is electrically connected to awiring which supplies a common potential.

The liquid crystal element 108 is an element which controls transmissionof light by an optical modulation action of liquid crystal which issandwiched between a substrate provided with the transistor 103 and thepixel electrode and a substrate provided with the counter electrode. Theoptical modulation action of liquid crystal is controlled by an electricfield applied to the liquid crystal (including a vertical electric fieldand a diagonal electric field). In the case where a counter electrode(also referred to as a common electrode) is provided over the substratewhere the pixel electrode is provided, an electric field applied toliquid crystal is a transverse electric field.

Next, a specific example of the pixel 101 of the liquid crystal displaydevice will be described. FIG. 2 is a top view of the pixel 101. Notethat in FIG. 2, the counter electrode and the liquid crystal element areomitted.

In FIG. 2, the scan line 107 is provided so as to extend in thedirection perpendicular or substantially perpendicular to the signalline 109 (in the horizontal direction in the drawing). The signal line109 is provided so as to extend in the direction perpendicular orsubstantially perpendicular to the scan line 107 (in the verticaldirection in the drawing). The capacitor line 115 is provided so as toextend in the direction parallel with the scan line 107. The scan line107 and the capacitor line 115 are electrically connected to the scanline driver circuit 104 (see FIG. 1A), and the signal line 109 iselectrically connected to the signal line driver circuit 106 (see FIG.1A).

The transistor 103 is provided in a region where the scan line 107 andthe signal line 109 cross each other. The transistor 103 includes atleast a semiconductor film 111 including a channel formation region, agate electrode, a gate insulating film (not illustrated in FIG. 2), asource electrode, and a drain electrode. A portion of the scan line 107which overlaps with the semiconductor film 111 functions as the gateelectrode of the transistor 103. A portion of the signal line 109 whichoverlaps with the semiconductor film 111 functions as the sourceelectrode of the transistor 103. A portion of a conductive film 113which overlaps with the semiconductor film 111 functions as the drainelectrode of the transistor 103. Thus, the gate electrode, the sourceelectrode, and the drain electrode may be referred to as the scan line107, the signal line 109, and the conductive film 113, respectively.Further, in FIG. 2, an edge of the scan line 107 is on the outer sidethan an edge of the semiconductor film when seen from above. Thus, thescan line 107 functions as a light-blocking film for blocking light froma light source such as a backlight. For this reason, the semiconductorfilm 111 included in the transistor is not irradiated with light, sothat variations in the electrical characteristics of the transistor canbe reduced.

Further, an oxide semiconductor processed under appropriate conditionscan significantly reduce the off-state current of a transistor;therefore, such an oxide semiconductor is used for the semiconductorfilm 111 in one embodiment of the present invention. Thus, powerconsumption of a semiconductor device can be reduced.

The conductive film 113 is electrically connected to a pixel electrode121 formed using a light-transmitting conductive film, through anopening 117. In FIG. 2, the hatch pattern of the pixel electrode 121 isnot illustrated.

The capacitor 105 is provided in a region of the pixel 101 and locatedon the inner sides of the capacitor lines 115 and the signal lines 109.The capacitor 105 is electrically connected to the capacitor line 115through a conductive film 125 provided in and over an opening 123. Thecapacitor 105 includes a semiconductor film 119 including an oxidesemiconductor, the pixel electrode 121, and an insulating film (notillustrated in FIG. 2) which is formed as a dielectric film over thetransistor 103. The semiconductor film 119, the pixel electrode 121, andthe dielectric film transmit light; accordingly, the capacitor 105transmits light.

Thanks to the light-transmitting property of the semiconductor film 119,the capacitor 105 can be formed large (in a large area) in the pixel101. Thus, a semiconductor device having charge capacity increased whileimproving the aperture ratio, to typically 55% or more, preferably 60%or more can be obtained. For example, in a semiconductor device with ahigh resolution such as a liquid crystal display device, the area of apixel is small and thus the area of a capacitor is also small. For thisreason, the capacity of charge stored in the capacitor is small.However, since the capacitor 105 of this embodiment transmits light,when it is provided in a pixel, enough charge capacity can be obtainedin the pixel and the aperture ratio can be improved. Typically, thecapacitor 105 can be favorably used in a high-resolution semiconductordevice with a pixel density of 200 ppi or more, or furthermore, 300 ppior more. Further, according to one embodiment of the present invention,the aperture ratio can be improved even in a display device with a highresolution, which makes it possible to use light from a light sourcesuch as a backlight efficiently, so that power consumption of thedisplay device can be reduced.

Here, the characteristics of a transistor including an oxidesemiconductor will be described. The transistor including an oxidesemiconductor is an n-channel transistor. Further, carriers might begenerated due to oxygen vacancies in the oxide semiconductor, whichmight degrade the electrical characteristics and reliability of thetransistor. For example, in some cases, the threshold voltage of thetransistor is shifted in the negative direction, and drain current flowswhen the gate voltage is 0 V. A transistor in which drain current flowswhen the gate voltage is 0 V is referred to as a normally-on transistor,whereas a transistor in which substantially no drain current flows whenthe gate voltage is 0 V is referred to as a normally-off transistor.

In view of the above, it is preferable that defects in an oxidesemiconductor film as the semiconductor film 111, typically, oxygenvacancies be reduced as much as possible when an oxide semiconductor isused for the semiconductor film 111. For example, it is preferable thatthe spin density of the oxide semiconductor film (the density of defectsin the oxide semiconductor film) at a g-value of 1.93 in electron spinresonance spectroscopy in which a magnetic field is applied in parallelwith the film surface be reduced to lower than or equal to the lowerdetection limit of measurement equipment. When the defects typified byoxygen vacancies in the oxide semiconductor film are reduced as much aspossible, the transistor 103 can be prevented from being normally on,leading to improvements in the electrical characteristics andreliability of a semiconductor device.

The shift of the threshold voltage of a transistor in the negativedirection is caused by hydrogen (including a hydrogen compound such aswater) contained in an oxide semiconductor in some cases as well as byoxygen vacancies. Hydrogen contained in the oxide semiconductor isreacted with oxygen bonded to a metal atom to be water, and in addition,vacancies (also referred to as oxygen vacancies) are formed in a latticefrom which oxygen is released (or a portion from which oxygen isremoved). In addition, the reaction of part of hydrogen and oxygencauses generation of electrons serving as carriers. Thus, a transistorincluding an oxide semiconductor which contains hydrogen is likely to benormally on.

In view of the above, when an oxide semiconductor is used for thesemiconductor film 111, it is preferable that hydrogen in the oxidesemiconductor film as the semiconductor film 111 be reduced as much aspossible. Specifically, the concentration of hydrogen in thesemiconductor film 111, which is measured by secondary ion massspectrometry (SIMS), is set to lower than 5×10¹⁸ atoms/cm³, preferablylower than or equal to 1×10¹⁸ atoms/cm³, more preferably lower than orequal to 5×10¹⁷ atoms/cm³, still more preferably lower than or equal to1×10¹⁶ atoms/cm³.

The concentration of alkali metals or alkaline earth metals in thesemiconductor film 111, which is measured by secondary ion massspectrometry (SIMS), is set to lower than or equal to 1×10¹⁸ atoms/cm³,preferably lower than or equal to 2×10¹⁶ atoms/cm³. This is because analkali metal and an alkaline earth metal might generate carriers whenbonded to an oxide semiconductor, in which case the off-state current ofthe transistor 103 might be increased.

Further, when nitrogen is contained in the oxide semiconductor film asthe semiconductor film 111, electrons serving as carriers are generatedand the carrier density increases, so that the oxide semiconductor filmeasily becomes n-type. Thus, a transistor including an oxidesemiconductor which contains nitrogen is likely to be normally on. Forthis reason, nitrogen in the oxide semiconductor film is preferablyreduced as much as possible; the concentration of nitrogen is preferablyset to, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

When such an oxide semiconductor film highly purified by reducingimpurities (such as hydrogen, nitrogen, an alkali metal, and an alkalineearth metal) as much as possible is used as the semiconductor film 111,the transistor 103 can be prevented from being normally on, so that theoff-state current of the transistor 103 can be significantly reduced.Therefore, a semiconductor device having favorable electricalcharacteristics can be fabricated. Further, a highly reliablesemiconductor device can be fabricated.

Various experiments can prove the low off-state current of a transistorincluding a highly-purified oxide semiconductor film. For example, evenwhen an element has a channel width of 1×10⁶ μm and a channel length (L)of 10 μm, the off-state current can be less than or equal to themeasurement limit of a semiconductor parameter analyzer, i.e., less thanor equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a sourceelectrode and a drain electrode of from 1 V to 10 V. In this case, itcan be seen that the off-state current corresponding to a value obtainedby dividing the off-state current by the channel width of the transistoris 100 zA/μm or lower. Further, the off-state current was measured withthe use of a circuit in which a capacitor and a transistor are connectedto each other and charge that flows in or out from the capacitor iscontrolled by the transistor. In the measurement, a purified oxidesemiconductor film was used for a channel formation region of thetransistor, and the off-state current of the transistor was measuredfrom a change in the amount of charge of the capacitor per unit time. Asa result, it is found that in the case where the voltage between asource electrode and a drain electrode of the transistor is 3 V, a loweroff-state current of several tens of yoctoamperes per micrometer (yA/μm)can be achieved. Thus, the transistor including the highly purifiedoxide semiconductor film has a significantly low off-state current.

Next, FIG. 3 is a cross-sectional view taken along dashed-dotted linesA1-A2 and B1-B2 in FIG. 2.

A cross-sectional structure of the pixel 101 of the liquid crystaldisplay device is as follows. The liquid crystal display device includesan element portion over a substrate 102, an element portion on asubstrate 150, and a liquid crystal layer sandwiched between the twoelement portions.

First, the structure of the element portion over the substrate 102 willbe described. The scan line 107 including a gate electrode 107 a of thetransistor 103 and the capacitor line 115 over the same surface as thescan line 107 are provided over the substrate 102. A gate insulatingfilm 127 is provided over the scan line 107 and the capacitor line 115.The semiconductor film 111 is provided over a portion of the gateinsulating film 127 which overlaps with the scan line 107, and thesemiconductor film 119 is provided over the gate insulating film 127.The signal line 109 including a source electrode 109 a of the transistor103 and the conductive film 113 including a drain electrode 113 a of thetransistor 103 are provided over the semiconductor film 111 and the gateinsulating film 127. An opening 123 reaching the capacitor line 115 isformed in the gate insulating film 127, and the conductive film 125 isprovided in and over the opening 123 and over the gate insulating film127 and the semiconductor film 119. An insulating film 129, aninsulating film 131, and an insulating film 132 functioning asprotective insulating films of the transistor 103 are provided over thegate insulating film 127, the signal line 109, the semiconductor film111, the conductive film 113, the conductive film 125, and thesemiconductor film 119. The opening 117 reaching the conductive film 113is formed in the insulating film 129, the insulating film 131, and theinsulating film 132, and the pixel electrode 121 is provided in theopening 117 and over the insulating film 132. An insulating film 158functioning as an alignment film is provided over the pixel electrode121 and the insulating film 132. Note that a base insulating film may beprovided between the substrate 102 and each of the scan line 107, thecapacitor line 115, and the gate insulating film 127.

In the capacitor 105 described in this embodiment, the semiconductorfilm 119 formed in a manner similar to that of the semiconductor film111 serves as one of a pair of electrodes, the pixel electrode 121serves as the other of the pair of electrodes, and the insulating film129, the insulating film 131, and the insulating film 132 serve as adielectric film provided between the pair of electrodes.

The details of the components of the above structure will be describedbelow.

Although there is no particular limitation on a material and the like ofthe substrate 102, it is necessary that the substrate have heatresistance high enough to withstand at least heat treatment performed ina fabrication process of a semiconductor device. Examples of thesubstrate are a glass substrate, a ceramic substrate, and a plasticsubstrate, and as the glass substrate, an alkali-free glass substratesuch as a barium borosilicate glass substrate, an aluminoborosilicateglass substrate, or an aluminosilicate glass substrate is preferablyused. Alternatively, a non-light-transmitting substrate such as astainless alloy substrate may be used, in which case a surface of thesubstrate is preferably provided with an insulating film. As thesubstrate 102, any of the following may alternatively be used: a quartzsubstrate, a sapphire substrate, a single crystal semiconductorsubstrate, a polycrystalline semiconductor substrate, a compoundsemiconductor substrate, and a silicon on insulator (SOI) substrate.

The scan line 107 and the capacitor line 115, through which a largeamount of current flows, are preferably formed using a metal film;typically, they are formed to have a single-layer structure or a layeredstructure using any of metal materials such as molybdenum (Mo), titanium(Ti), tungsten (W), tantalum (Ta), aluminum (Al), copper (Cu), chromium(Cr), neodymium (Nd), or scandium (Sc), or an alloy material whichcontains any of these materials as its main component.

Examples of the scan line 107 and the capacitor line 115 are asingle-layer structure using aluminum containing silicon, a two-layerstructure in which titanium is stacked over aluminum, a two-layerstructure in which titanium is stacked over titanium nitride, atwo-layer structure in which tungsten is stacked over titanium nitride,a two-layer structure in which tungsten is stacked over tantalumnitride, a two-layer structure in which copper is stacked over Cu—Mg—Alalloy, and a three-layer structure in which titanium nitride, copper,and tungsten are stacked in this order.

As a material of the scan line 107 and the capacitor line 115, alight-transmitting conductive material which can be used for the pixelelectrode 121 can be used.

Alternatively, as a material of the scan line 107 and the capacitor line115, a metal oxide containing nitrogen, specifically, an In—Ga—Zn-basedoxide containing nitrogen, an In—Sn-based oxide containing nitrogen, anIn—Ga-based oxide containing nitrogen, an In—Zn-based oxide containingnitrogen, a Sn-based oxide containing nitrogen, an In-based oxidecontaining nitrogen, or a metal nitride (InN, SnN, or the like) can beused. These materials each have a work function higher than or equal to5 eV (electron volts). When such an oxide semiconductor is used for thesemiconductor film 111 in the transistor 103, the use of a metal oxidecontaining nitrogen for the scan line 107 (the gate electrode of thetransistor 103) allows the threshold voltage of the transistor 103 to beshifted in the positive direction, i.e., the transistor can be normallyoff. For example, in the case of using an In—Ga—Zn-based oxidecontaining nitrogen, an In—Ga—Zn-based oxide having at least a highernitrogen concentration than the semiconductor film 111, specifically, anIn—Ga—Zn-based oxide having a nitrogen concentration of 7 at. % orhigher can be used.

The scan line 107 and the capacitor line 115 are preferably formed usingaluminum or copper, which are low resistance materials. With the use ofaluminum or copper, signal delay is reduced, so that higher imagequality can be achieved. Note that aluminum has low heat resistance, andthus a defect due to hillocks, whiskers, or migration is easilygenerated. To prevent migration of aluminum, a layer of a metal materialhaving a higher melting point than aluminum, such as molybdenum,titanium, or tungsten, is preferably stacked over an aluminum layer.Also in the case where copper is used, in order to prevent a defect dueto migration and diffusion of copper element, a layer of a metalmaterial having a higher melting point than copper, such as molybdenum,titanium, or tungsten, is preferably stacked over a copper layer.

The gate insulating film 127 is formed to have a single-layer structureor a layered structure using, for example, any of insulating materialssuch as silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, and aGa—Zn-based metal oxide. In order to improve the characteristics of theinterface between the gate insulating film 127 and the oxidesemiconductor film as the semiconductor film 111, a region in the gateinsulating film 127 which is in contact with at least the semiconductorfilm 111 is preferably formed using an oxide insulating film.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film as the semiconductor film 111 and entry ofhydrogen, water, or the like into the oxide semiconductor film from theoutside by providing an insulating film having a barrier propertyagainst oxygen, hydrogen, water, and the like under the gate insulatingfilm 127. Examples of the insulating film having a barrier propertyagainst oxygen, hydrogen, water, and the like are an aluminum oxidefilm, an aluminum oxynitride film, a gallium oxide film, a galliumoxynitride film, an yttrium oxide film, an yttrium oxynitride film, ahafnium oxide film, a hafnium oxynitride film, and a silicon nitridefilm.

The gate insulating film 127 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate containing nitrogen(HfSi_(x)O_(y)N_(z)), hafnium aluminate containing nitrogen(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, in which casegate leakage current of the transistor 103 can be reduced.

The gate insulating film 127 preferably has the following layeredstructure. It is preferable that a silicon nitride film having fewerdefects be provided as a first silicon nitride film, a silicon nitridefilm from which less hydrogen and ammonia are released be provided as asecond silicon nitride film over the first silicon nitride film, and anyof the oxide insulating films listed as those used for the gateinsulating film 127 be provided over the second silicon nitride film.

As the second silicon nitride film, a nitride insulating film whichreleases hydrogen molecules less than 5×10²¹ molecules/cm³, preferablyless than or equal to 3×10²¹ molecules/cm³, more preferably less than orequal to 1×10²¹ molecules/cm³, and ammonia molecules less than 1×10²²molecules/cm³, preferably less than or equal to 5×10²¹ molecules/cm³,more preferably less than or equal to 1×10²¹ molecules/cm³ by thermaldesorption spectroscopy is preferably used. The first silicon nitridefilm and the second silicon nitride film are used as part of the gateinsulating film 127, whereby a gate insulating film which has fewerdefects and from which less hydrogen and ammonia are released can beformed as the gate insulating film 127. Thus, the amount of hydrogen andnitrogen contained in the gate insulating film 127 which enter thesemiconductor film 111 can be reduced.

In the case where the trap level (also referred to as interface level)is present at the interface between an oxide semiconductor film and agate insulating film or in the gate insulating film in a transistorincluding an oxide semiconductor, a shift of the threshold voltage ofthe transistor, typically, a shift of the threshold voltage in thenegative direction, and an increase in the subthreshold swing (S value)showing a gate voltage needed for changing the drain current by an orderof magnitude when the transistor is turned on are caused. As a result,there is a problem in that electrical characteristics vary amongtransistors. Therefore, the use of a silicon nitride film having fewerdefects as a gate insulating film and provision of an oxide insulatingfilm in contact with the semiconductor film 111 can reduce a shift ofthe threshold voltage in the negative direction and minimize an increasein S value.

The thickness of the gate insulating film 127 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, more preferably greaterthan or equal to 50 nm and less than or equal to 250 nm.

The semiconductor film 111 and the semiconductor film 119 are oxidesemiconductor films which can be amorphous, single-crystalline, orpolycrystalline. Further, the semiconductor film 111 and thesemiconductor film 119 are formed using the same metal element. Thethickness of the semiconductor film 111 is greater than or equal to 1 nmand less than or equal to 100 nm, preferably greater than or equal to 1nm and less than or equal to 50 nm, more preferably greater than orequal to 1 nm and less than or equal to 30 nm, still more preferablygreater than or equal to 3 nm and less than or equal to 20 nm.

An oxide semiconductor which can be used for the semiconductor film 111and the semiconductor film 119 has an energy gap of greater than orequal to 2 eV, preferably greater than or equal to 2.5 eV, morepreferably greater than or equal to 3 eV. The use of such an oxidesemiconductor having a wide energy gap can reduce the off-state currentof the transistor 103.

An oxide semiconductor used for the semiconductor film 111 is preferablya metal oxide containing at least indium (In) or zinc (Zn).Alternatively, the oxide semiconductor is preferably a metal oxidecontaining both In and Zn. In order to reduce variations in electricalcharacteristics of the transistors including the oxide semiconductor,the oxide semiconductor preferably contains one or more stabilizers inaddition to one of or both In and Zn.

Examples of stabilizers are gallium (Ga), tin (Sn), hafnium (Hf),aluminum (Al), and zirconium (Zr). The other examples of stabilizers arelanthanoids such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu).

For an oxide semiconductor which can be used for the semiconductor film111 and the semiconductor film 119, for example, the following can beused: an indium oxide; a tin oxide; a zinc oxide; an oxide containingtwo kinds of metals, such as an In—Zn-based oxide, a Sn—Zn-based oxide,an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, anIn—Mg-based oxide, or an In—Ga-based oxide; an oxide containing threekinds of metals, such as an In—Ga—Zn-based oxide (also referred to asIGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, aSn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—Zr—Zn-based oxide, an In—Ti—Zn-basedoxide, an In—Sc—Zn-based oxide, an In—Y—Zn-based oxide, anIn—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide,an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-basedoxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, anIn—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide,an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-basedoxide; or an oxide containing four kinds of metals, such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theratio of In, Ga, and Zn. Further, the In—Ga—Zn-based oxide may contain ametal element other than In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0) may beused as an oxide semiconductor. Note that M represents one or more metalelements selected from Ga, Fe, Mn, and Co, or the above element as astabilizer.

For example, an In—Ga—Zn-based metal oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=⅓:⅓:⅓), In:Ga:Zn=2:2:1 (=⅖:⅖:⅕), or In:Ga:Zn=3:1:2(=½:⅙:⅓). Alternatively, an In—Sn—Zn-based oxide with an atomic ratio ofIn:Sn:Zn=1:1:1 (=⅓:⅓:⅓), In:Sn:Zn=2:1:3 (=⅓:⅙:½), or In:Sn:Zn=2:1:5(=¼:⅛:⅝) may be used. Note that the proportion of each atom in theatomic ratio of the metal oxide varies within a range of ±20% as anerror.

Note that without limitation to the materials given above, a materialwith an appropriate atomic ratio depending on semiconductorcharacteristics and electrical characteristics (field-effect mobility,threshold voltage, variation, and the like) may be used. Further, it ispreferable to appropriately set the carrier density, the impurityconcentration, the defect density, the atomic ratio of a metal elementand oxygen, the interatomic distance, the density, or the like in orderto obtain necessary semiconductor characteristics. For example, highfield-effect mobility can be achieved relatively easily in the case ofusing an In—Sn—Zn oxide. Also in the case of using an In—Ga—Zn-basedoxide, field-effect mobility can be increased by reducing the defectdensity in a bulk.

The signal line 109 including the source electrode 109 a of thetransistor 103, the conductive film 113 including the drain electrode ofthe transistor 103, and the conductive film 125 electrically connectingthe semiconductor film 119 and the capacitor line 115 in the capacitor105 can be formed to have a single-layer structure or a layeredstructure using a material which can be used for the scan line 107 andthe capacitor line 115.

The insulating films 129, 131, and 132 functioning as the protectiveinsulating films of the transistor 103 and the dielectric film in thecapacitor 105 are insulating films each formed using a material whichcan be used for the gate insulating film 127. It is particularlypreferable that the insulating films 129 and 131 be oxide insulatingfilms and the insulating film 132 be a nitride insulating film. Further,the use of a nitride insulating film as the insulating film 132 cansuppress entry of impurities such as hydrogen and water into thetransistor 103 (in particular, the semiconductor film 111) from theoutside. Note that the insulating film 129 is not necessarily provided.

Further, an oxide insulating film in which the oxygen content is higherthan that in the stoichiometric composition is preferably used as one ofor both the insulating film 129 and the insulating film 131. In thatcase, oxygen can be prevented from being released from the oxidesemiconductor film, and the oxygen contained in the oxide insulatingfilm can enter the oxide semiconductor film to reduce oxygen vacancies.For example, when an oxide insulating film having the following featureis used, oxygen vacancies in the oxide semiconductor film can bereduced. The feature of the oxide insulating film is that the number ofoxygen molecules released from the oxide insulating film is greater thanor equal to 1.0×10¹⁸/cm³ when measured by thermal desorptionspectroscopy (hereinafter referred to as TDS spectroscopy). Note that anoxide insulating film partly including a region in which the oxygencontent is higher than that in the stoichiometric composition (oxygenexcess region) may be used as one of or both the insulating film 129 andthe insulating film 131. When such an oxygen excess region is present ina region overlapping with at least the semiconductor film 111, oxygen isprevented from being released from the oxide semiconductor film and theoxygen contained in the oxygen excess region can enter the oxidesemiconductor film to reduce oxygen vacancies.

In the case where the insulating film 131 is an oxide insulating film inwhich the oxygen content is higher than that in the stoichiometriccomposition, the insulating film 129 is preferably an oxide insulatingfilm through which oxygen penetrates. Oxygen which enters the insulatingfilm 129 from the outside does not completely penetrate through theinsulating film 129 to be released and part thereof remains in theinsulating film 129. Further, there is oxygen which is contained in theinsulating film 129 from the first and is released from the insulatingfilm 129 to the outside. Thus, the insulating film 129 preferably has ahigh coefficient of diffusion of oxygen.

Since the insulating film 129 is in contact with the oxide semiconductorfilm as the semiconductor film 111, the insulating film 129 ispreferably an oxide insulating film through which oxygen penetrates andwhich has a low interface state with the semiconductor film 111. Forexample, the insulating film 129 is preferably an oxide insulating filmhaving a lower defect density than the insulating film 131.Specifically, the spin density of the oxide insulating film at a g-valueof 2.001 (E′-center) measured by electron spin resonance spectroscopy islower than or equal to 3.0×10¹⁷ spins/cm³, preferably lower than orequal to 5.0×10¹⁶ spins/cm³. The spin density at a g-value of 2.001measured by electron spin resonance spectroscopy corresponds to thenumber of dangling bonds in the insulating film 129.

The insulating film 129 can have a thickness of greater than or equal to5 nm and less than or equal to 150 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm, more preferably greater than orequal to 10 nm and less than or equal to 30 nm. The insulating film 131can have a thickness of greater than or equal to 30 nm and less than orequal to 500 nm, preferably greater than or equal to 150 nm and lessthan or equal to 400 nm.

In the case where a nitride insulating film is used as the insulatingfilm 132, an insulating film having a barrier property against nitrogenis preferably used as one of or both the insulating film 129 and theinsulating film 131. For example, a dense oxide insulating film can havea barrier property against nitrogen. Specifically, an oxide insulatingfilm which can be etched at a rate of less than or equal to 10 nm perminute when the temperature is 25° C. and 0.5 wt % of fluoric acid isused is preferably used.

In the case where an oxide insulating film containing nitrogen, such asa silicon oxynitride film or a silicon nitride oxide film, is used asone of or both the insulating film 129 and the insulating film 131, thenitrogen concentration measured by SIMS is greater than or equal to thelower limit of measurement by SIMS and less than 3×10²⁰ atoms/cm³,preferably greater than or equal to 1×10¹⁸ atoms/cm³ and less than orequal to 1×10²⁰ atoms/cm³. In that case, the amount of nitrogen whichenters the semiconductor film 111 included in the transistor 103 can bereduced and the number of defects in the nitrogen-containing oxideinsulating film itself can be reduced.

As the insulating film 132, a nitride insulating film where the hydrogencontent is low may be provided. The nitride insulating film is asfollows, for example: the number of hydrogen molecules released from thenitride insulating film is less than 5.0×10²¹/cm³, preferably less than3.0×10²¹/cm³, more preferably less than 1.0×10²¹/cm³ when measured byTDS spectroscopy.

The insulating film 132 has a thickness large enough to prevent entry ofimpurities such as hydrogen and water from the outside. For example, thethickness can be greater than or equal to 50 nm and less than or equalto 200 nm, preferably greater than or equal to 50 nm and less than orequal to 150 nm, more preferably greater than or equal to 50 nm and lessthan or equal to 100 nm.

Further, a silicon oxide film formed by a CVD method using anorganosilane gas may be provided between the insulating film 131 and theinsulating film 132. The silicon oxide film has excellent step coverageand thus can be advantageously used as a protective insulating film ofthe transistor 103. The silicon oxide film can be formed to a thicknessof 300 nm to 600 nm inclusive. As the organosilane gas, any of thefollowing silicon-containing compound can be used: tetraethylorthosilicate (TEOS) (chemical formula: Si(OC₂H₅)₄); tetramethylsilane(TMS) (chemical formula: Si(CH₃)₄); tetramethylcyclotetrasiloxane(TMCTS); octamethylcyclotetrasiloxane (OMCTS); hexamethyldisilazane(HMDS); triethoxysilane (SiH(OC₂H₅)₃); trisdimethylaminosilane(SiH(N(CH₃)₂)₃); or the like.

According to the above description, when the silicon oxide film isprovided between the insulating film 131 and the insulating film 132 andthe nitride insulating film is used as the insulating film 132, entry ofimpurities such as hydrogen and water into the semiconductor film 111and the semiconductor film 119 from the outside can be furthersuppressed.

The pixel electrode 121 is formed using a light-transmitting conductivefilm. The light-transmitting conductive film is formed using alight-transmitting conductive material such as an indium tin oxide, anindium oxide containing a tungsten oxide, an indium zinc oxidecontaining a tungsten oxide, an indium oxide containing a titaniumoxide, an indium tin oxide containing a titanium oxide, an indium zincoxide, or an indium tin oxide to which a silicon oxide is added.

Next, the structure of the element portion on the substrate 150 will bedescribed. The element portion includes a light-blocking film 152 whichis in contact with the substrate 150, an electrode (a counter electrode154) which is in contact with the light-blocking film 152 and isprovided so as to face the pixel electrode 121, and an insulating film156 which is in contact with the counter electrode 154 and functions asan alignment film.

The light-blocking film 152 prevents the transistor 103 from beingirradiated with light from a light source such as a backlight or theoutside. The light-blocking film 152 can be formed using a material suchas a metal or an organic resin including a pigment and may be providedin a region outside the pixel portion 100, such as over the scan linedriver circuit 104 and over the signal line driver circuit 106 (see FIG.1A), as well as over the transistor 103 in the pixel 101.

Note that a coloring film which transmits light with a predeterminedwavelength may be provided across a space between light-blocking films152 adjacent to each other. Further, an overcoat film may be providedbetween the counter electrode 154, and the light-blocking films 152 andthe coloring film.

The counter electrode 154 is formed using any of the light-transmittingconductive materials given as those used for the pixel electrode 121 asappropriate.

The liquid crystal element 108 includes the pixel electrode 121, thecounter electrode 154, and a liquid crystal layer 160. The liquidcrystal layer 160 is sandwiched between the insulating film 158 which isprovided in the element portion over the substrate 102 and functions asan alignment film and the insulating film 156 which is provided in theelement portion on the substrate 150 and functions as an alignment film.Further, the pixel electrode 121 overlaps with the counter electrode 154with the liquid crystal layer 160 interposed therebetween.

The insulating films 156 and 158 functioning as alignment films can beformed using a general-purpose material such as polyamide.

Here, connection of the components included in the pixel 101 describedin this embodiment will be described with reference to the circuitdiagram in FIG. 1C and the cross-sectional view in FIG. 3.

FIG. 1C is an example of a detailed circuit diagram of the pixel 101included in the semiconductor device illustrated in FIG. 1A. Asillustrated in FIG. 1C and FIG. 3, the transistor 103 includes the scanline 107 including the gate electrode 107 a, the signal line 109including the source electrode 109 a, and the conductive film 113including the drain electrode 113 a.

In the capacitor 105, the semiconductor film 119 connected to thecapacitor line 115 through the conductive film 125 functions as oneelectrode; the pixel electrode 121 connected to the conductive film 113including the drain electrode 113 a functions as the other electrode;and the insulating films 129, 131, and 132 provided between thesemiconductor film 119 and the pixel electrode 121 function as adielectric film.

The liquid crystal element 108 includes the pixel electrode 121, thecounter electrode 154, and the liquid crystal layer 160 provided betweenthe pixel electrode 121 and the counter electrode 154.

Despite having a structure which is the same as that of thesemiconductor film 111, the semiconductor film 119 in the capacitor 105functions as the electrode of the capacitor 105. This is because thepixel electrode 121 can function as a gate electrode, the insulatingfilms 129, 131, and 132 can function as gate insulating films, and acapacitor line 315 can function as a source electrode or a drainelectrode, so that the capacitor 105 can be operated in a manner similarto that of a transistor and the semiconductor film 119 can be made to bein a conductive state. In other words, the capacitor 105 can be a metaloxide semiconductor (MOS) capacitor. Power is supplied to a MOScapacitor when a voltage higher than the threshold voltage (Vth) isapplied to one electrode of the MOS capacitor (the pixel electrode 121of the capacitor 105) as shown in FIG. 38. In FIG. 38, the horizontalaxis indicates voltage (V) applied to the pixel electrode, and thelongitudinal axis indicates capacitance (C). In the case where thefrequency of a voltage in capacitance-voltage measurement (CVmeasurement) is lower than frame frequency, a CV curve in FIG. 38 isobtained, i.e., the threshold voltage Vth is higher than or equal to 0V. Further, the semiconductor film 119 can be made to be in a conductivestate so that the semiconductor film 119 can function as one electrodeof the capacitor by controlling a potential to be supplied to thecapacitor line 115. In this case, the potential to be supplied to thecapacitor line 115 is set as follows as in FIG. 39A. The potential ofthe pixel electrode 121 is changed in the positive direction and thenegative direction relative to the medium potential of a video signal inorder to operate the liquid crystal element 108 (see FIG. 1C). Thepotential (VCs) of the capacitor line 115 needs to be constantly lowerthan the potential to be supplied to the pixel electrode 121 by thethreshold voltage (Vth) of the capacitor 105 (MOS capacitor) or more inorder that the capacitor 105 (MOS capacitor) be constantly in aconductive state. In other words, since the semiconductor film 119 hasthe same structure as the semiconductor film 111, the potential (VCs) ofthe capacitor line 115 should be lower than the potential to be suppliedto the pixel electrode 121 by the threshold voltage of the transistor103 or more. In such a manner, the semiconductor film 119 can be made tobe constantly in a conductive state. In FIGS. 39A and 39B, GVss refersto a low-level potential to be supplied to the gate electrode and GVddrefers to a high-level potential to be supplied to the gate electrode toturn on the transistor 103.

When an oxide insulating film through which oxygen penetrates and whichhas fewer interface states between the semiconductor film 111 and theoxide insulating film is used as the insulating film 129 over thesemiconductor film 111 and an oxide insulating film which includes anoxygen excess region or an oxide insulating film in which the oxygencontent is higher than that in the stoichiometric composition is used asthe insulating film 131, oxygen can be easily supplied to the oxidesemiconductor film as the semiconductor film 111, the release of oxygenfrom the oxide semiconductor film can be prevented, and the oxygencontained in the insulating film 131 can enter the oxide semiconductorfilm to reduce oxygen vacancies in the oxide semiconductor film. Thus,the transistor 103 can be prevented from being normally on and apotential to be supplied to the capacitor line 115 can be controlled sothat the capacitor 105 (MOS capacitor) can be constantly in a conductivestate; thus, the semiconductor device can have favorable electricalcharacteristics and high reliability.

The use of a nitride insulating film as the insulating film 132 over theinsulating film 131 can suppress entry of impurities such as hydrogenand water into the semiconductor film 111 and the semiconductor film 119from the outside. Moreover, the use of a nitride insulating film with alow hydrogen content as the insulating film 132 can minimize variationsin electrical characteristics of the transistor 103 and the capacitor105 (MOS capacitor).

Further, the capacitor 105 can be formed large (in a large area) in thepixel 101. Thus, the semiconductor device can have charge capacityincreased while improving the aperture ratio. As a result, thesemiconductor device can have an excellent display quality.

<Fabrication Method of Semiconductor Device>

Next, a formation method of the element portion over the substrate 102in the semiconductor device described above will be described withreference to FIGS. 4A and 4B and FIGS. 5A and 5B.

First, the scan line 107 and the capacitor line 115 are formed over thesubstrate 102. An insulating film 126 which is to be processed into thegate insulating film 127 later is formed so as to cover the scan line107 and the capacitor line 115. The semiconductor film 111 is formedover a portion of the insulating film 126 which overlaps with the scanline 107. The semiconductor film 119 is formed so as to overlap a regionwhere the pixel electrode 121 is to be formed later (see FIG. 4A).

The scan line 107 and the capacitor line 115 can be formed in such amanner that a conductive film is formed using any of the materials givenabove, a mask is formed over the conductive film, and processing isperformed using the mask. The conductive film can be formed by any of avariety of deposition methods such as an evaporation method, a CVDmethod, a sputtering method, and a spin coating method. Note that thethickness of the conductive film is not particularly limited and can bedetermined in consideration of formation time, desired resistivity, andthe like. As the mask, a resist mask formed through a firstphotolithography process can be used. The conductive film can beprocessed by one of or both dry etching and wet etching.

The insulating film 126 can be formed using a material which can be usedfor the gate insulating film 127, by any of a variety of depositionmethods such as a CVD method and a sputtering method.

In the case where a gallium oxide is used for the gate insulating film127, the insulating film 126 can be formed by a metal organic chemicalvapor deposition (MOCVD) method.

The semiconductor film 111 and the semiconductor film 119 can be formedin such a manner that any of the oxide semiconductor films given aboveis appropriately selected and formed, a mask is formed over the formedoxide semiconductor film, and processing is performed using the mask.Thus, the semiconductor film 111 and the semiconductor film 119 areformed using the same metal element. The oxide semiconductor film can beformed by a sputtering method, a coating method, a pulsed laserdeposition method, a laser ablation method, or the like. Alternatively,when a printing method is employed, the semiconductor films 111 and 119which are separate from each other can be formed directly on theinsulating film 126. In the case where the oxide semiconductor film isformed by a sputtering method, an RF power supply device, an AC powersupply device, a DC power supply device, or the like can be used asappropriate as a power supply device for generating plasma. As asputtering as, a rare gas (typically argon), an oxygen gas, or a mixedgas of a rare gas and oxygen is used as appropriate. In the case ofusing the mixed gas of a rare gas and oxygen, the proportion of oxygenis preferably higher than that of the rare gas. Further, a target may beappropriately selected in accordance with the composition of an oxidesemiconductor film which is to be faulted. As the mask, a resist maskformed through a second photolithography process can be used. The oxidesemiconductor film can be processed by one of or both dry etching andwet etching. Etching conditions (an etching gas, an etchant, etchingtime, temperature, and the like) are appropriately set in accordancewith a material so that etching can be performed to form a desiredshape.

Heat treatment is preferably performed after formation of thesemiconductor films 111 and 119 to dehydrate or dehydrogenate the oxidesemiconductor films as the semiconductor films 111 and 119. Thetemperature of the heat treatment is typically higher than or equal to150° C. and lower than the strain point of the substrate, preferablyhigher than or equal to 200° C. and lower than or equal to 450° C., morepreferably higher than or equal to 300° C. and lower than or equal to450° C. Note that the heat treatment may be performed on the oxidesemiconductor film which has not been processed into the semiconductorfilms 111 and 119.

A heat treatment apparatus used in the heat treatment is not limited toan electric furnace; as the heat treatment apparatus, an apparatus whichheats an object using thermal conduction or thermal radiation given by amedium such as a heated gas may be used. For example, a rapid thermalannealing (RTA) apparatus such as a gas rapid thermal annealing (GRTA)apparatus or a lamp rapid thermal annealing (LRTA) apparatus can beused. An LRTA apparatus is an apparatus for heating an object to beprocessed by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is less than orequal to 20 ppm, preferably less than or equal to 1 ppm, more preferablyless than or equal to 10 ppb), or a rare gas (e.g., argon or helium).The atmosphere of nitrogen, oxygen, ultra-dry air, or a rare gaspreferably does not contain hydrogen, water, and the like.Alternatively, heating may be performed in an inert gas atmospherefirst, and then in an oxygen atmosphere. The treatment time is threeminutes to 24 hours.

In the case where a base insulating film is provided between thesubstrate 102, and the scan line 107, the capacitor line 115, and thegate insulating film 127, the base insulating film can be formed usingany of the following: silicon oxide, silicon oxynitride, siliconnitride, silicon nitride oxide, gallium oxide, hafnium oxide, yttriumoxide, aluminum oxide, aluminum oxynitride, and the like. The use ofsilicon nitride, gallium oxide, hafnium oxide, yttrium oxide, aluminumoxide, or the like for the base insulating film leads to suppression ofdiffusion of impurities typified by an alkali metal, water, and hydrogeninto the semiconductor film 111 from the substrate 102. The baseinsulating film can be formed by a sputtering method or a CVD method.

After an opening 123 reaching the capacitor line 115 is formed in theinsulating film 126 to form the gate insulating film 127, the signalline 109 including the source electrode of the transistor 103, theconductive film 113 including the drain electrode of the transistor 103,and the conductive film 125 which electrically connects thesemiconductor film 119 and the capacitor line 115 are formed (see FIG.4B).

The opening 123 can be formed so as to expose part of a portion of theinsulating film 126 which overlaps with the capacitor line 115 in such amanner that a mask is formed through a third photolithography processand processing is performed using the mask. The formation of the maskand the processing can be performed in manners similar to those of thescan line 107 and the capacitor line 115.

The signal line 109, the conductive film 113, and the conductive film125 can be formed in such a manner that a conductive film is formedusing a material which can be used for the signal line 109, theconductive film 113, and the conductive film 125, a mask is formed overthe conductive film through a fourth photolithography process, andprocessing is performed using the mask.

Then, an insulating film 128 is formed over the semiconductor film 111,the semiconductor film 119, the signal line 109, the conductive film113, the conductive film 125, and the gate insulating film 127, aninsulating film 130 is formed over the insulating film 128, and aninsulating film 133 is formed over the insulating film 130 (see FIG.5A). The insulating film 128, the insulating film 130, and theinsulating film 133 are preferably formed successively, in which caseentry of impurities into each interface can be suppressed.

The insulating film 128 can be formed using a material which can be usedfor the insulating film 129, by any of a variety of deposition methodssuch as a CVD method and a sputtering method. The insulating film 130can be formed using a material which can be used for the insulating film131. The insulating film 133 can be formed using a material which can beused for the insulating film 132.

In the case where an oxide insulating film which has fewer interfacestates between the semiconductor film 111 and the oxide insulating filmis used as the insulating film 129, the insulating film 128 can beformed under the following formation conditions. Here, as the oxideinsulating film, a silicon oxide film or a silicon oxynitride film isformed. As for the formation conditions, the substrate placed in atreatment chamber of a plasma CVD apparatus, which is vacuum-evacuated,is held at a temperature higher than or equal to 180° C. and lower thanor equal to 400° C., preferably higher than or equal to 200° C. andlower than or equal to 370° C., a deposition gas containing silicon andan oxidizing gas are introduced as a source gas into the treatmentchamber, the pressure in the treatment chamber is greater than or equalto 20 Pa and less than or equal to 250 Pa, preferably greater than orequal to 40 Pa and less than or equal to 200 Pa, and high-frequencypower is supplied to an electrode provided in the treatment chamber.

Typical examples of the deposition gas containing silicon are silane,disilane, trisilane, and silane fluoride. Examples of the oxidizing gasare oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide.

By setting the ratio of the amount of the oxidizing gas to the amount ofthe deposition gas containing silicon to 100 or higher, the hydrogencontent in the insulating film 128 (the insulating film 129) can bereduced and dangling bonds in the insulating film 128 (the insulatingfilm 129) can be reduced. Oxygen released from the insulating film 130(the insulating film 131) is captured by the dangling bonds in theinsulating film 128 (the insulating film 129) in some cases; thus, inthe case where the dangling bonds in the insulating film 128 (theinsulating film 129) are reduced, oxygen in the insulating film 130 (theinsulating film 131) can enter the semiconductor film 111 and thesemiconductor film 119 efficiently to reduce the oxygen vacancies in theoxide semiconductor films as the semiconductor film 111 and thesemiconductor film 119. As a result, the amount of hydrogen which entersthe oxide semiconductor films can be reduced and oxygen vacancies in theoxide semiconductor films can be reduced.

In the case where the above oxide insulating film which includes anoxygen excess region or the above oxide insulating film in which theoxygen content is higher than that in the stoichiometric composition isused as the insulating film 131, the insulating film 130 can be formedunder the following formation conditions. Here, as the oxide insulatingfilm, a silicon oxide film or a silicon oxynitride film is formed. Asfor the formation conditions, the substrate placed in a treatmentchamber of a plasma CVD apparatus, which is vacuum-evacuated, is held ata temperature higher than or equal to 180° C. and lower than or equal to260° C., preferably higher than or equal to 180° C. and lower than orequal to 230° C., a source gas is introduced into the treatment chamber,the pressure in the treatment chamber is greater than or equal to 100 Paand less than or equal to 250 Pa, preferably greater than or equal to100 Pa and less than or equal to 200 Pa, and high-frequency power thatis higher than or equal to 0.17 W/cm² and lower than or equal to 0.5W/cm², preferably, higher than or equal to 0.25 W/cm² and lower than orequal to 0.35 W/cm² is supplied is supplied to an electrode provided inthe treatment chamber.

As the source gas of the insulating film 130, a source gas which can beused for the insulating film 128 can be used.

As for the formation conditions of the insulating film 130, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure, whereby the decompositionefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas proceeds; therefore, theoxygen content in the insulating film 130 is higher than that in thestoichiometric composition. On the other hand, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film in which the oxygen content ishigher than that in the stoichiometric composition and from which partof oxygen is released by heating. The insulating film 128 is providedover the semiconductor film 111. Accordingly, in the process for formingthe insulating film 130, the insulating film 128 serves as a protectivefilm of the semiconductor film 111. Thus, even when the insulating film130 is formed using the high-frequency power having a high powerdensity, damage to the semiconductor film 111 is not significant.

By increasing the thickness of the insulating film 130, a larger amountof oxygen is released by heating; thus, the insulating film 130 ispreferably formed thicker than the insulating film 128. Since theinsulating film 128 is provided, favorable coverage can be achieved evenwhen the insulating film 130 is formed thick.

In the case where a nitride insulating film with a low hydrogen contentis used as the insulating film 132, the insulating film 133 can beformed under the following formation conditions. Here, as the nitrideinsulating film, a silicon nitride film is formed. As for the formationconditions, the substrate placed in a treatment chamber of a plasma CVDapparatus, which is vacuum-evacuated, is held at a temperature higherthan or equal to 180° C. and lower than or equal to 400° C., preferablyhigher than or equal to 200° C. and lower than or equal to 370° C., asource gas is introduced into the treatment chamber, the pressure in thetreatment chamber is greater than or equal to 100 Pa and less than orequal to 250 Pa, preferably greater than or equal to 100 Pa and lessthan or equal to 200 Pa, and high-frequency power is supplied to anelectrode provided in the treatment chamber.

As the source gas of the insulating film 133, a deposition gascontaining silicon, a nitrogen gas, and an ammonia gas are preferablyused. Typical examples of the deposition gas containing silicon aresilane, disilane, trisilane, and silane fluoride. Further, the flowratio of nitrogen to ammonia is preferably higher than or equal to 5 andlower than or equal to 50, more preferably higher than or equal to 10and lower than or equal to 50. The use of ammonia as the source gasfacilitates decomposition of nitrogen and the deposition gas containingsilicon. This is because ammonia is dissociated by plasma energy or heatenergy, and energy generated by the dissociation contributes todecomposition of a bond of the deposition gas molecules containingsilicon and a bond of nitrogen molecules. Under the above conditions, asilicon nitride film which has a low hydrogen content and can suppressentry of impurities such as hydrogen and water from the outside can beformed.

Note that a silicon oxide film may be formed between the insulating film130 and the insulating film 133 by a CVD method using an organosilanegas.

It is preferable that heat treatment be performed at least afterformation of the insulating film 130 so that excess oxygen contained inthe insulating film 128 or the insulating film 130 enters thesemiconductor film 111 to reduce oxygen vacancies in the oxidesemiconductor film as the semiconductor film 111. The heat treatment canbe appropriately performed according to the details of heat treatmentfor dehydration or dehydrogenation of the semiconductor film 111 and thesemiconductor film 119.

In the case where a silicon oxide film is formed between the insulatingfilm 130 and the insulating film 133 by a CVD method using anorganosilane gas, an oxide insulating film in which the oxygen contentis higher than that in the stoichiometric composition and from whichpart of oxygen is released by heating is formed as the insulating film130 and then heat treatment is performed at 350° C. so that excessoxygen contained in the insulating film 130 enters the semiconductorfilm 111. After the silicon oxide film is formed by a CVD method usingany of the organosilane gases given above at a substrate temperature of350° C., a nitride insulating film with a low hydrogen content is formedas the insulating film 133 at a substrate temperature of 350° C.

Then, after a mask is formed over portions of the insulating film 128,the insulating film 130, and the insulating film 133 which overlap theconductive film 113 through a fifth photolithography process, theinsulating film 128, the insulating film 130, and the insulating film133 are etched to form the opening 117 reaching the conductive film 113(see FIG. 5B). The opening 117 can be formed in a manner similar to thatof the opening 123.

Finally, the pixel electrode 121 is formed, so that the element portionover the substrate 102 can be formed (see FIG. 3). The pixel electrode121 is formed in such a manner that a conductive film is formed usingany of the materials listed above in contact with the conductive film113 through the opening 117, a mask is formed over the conductive filmthrough a sixth photolithography process, and processing is performedusing the mask. The formation of the mask and the processing can beperformed in manners similar to those of the scan line 107 and thecapacitor line 115.

Modification Example 1

In the semiconductor device of one embodiment of the present invention,connection of the capacitor line and the semiconductor film serving asone electrode of the capacitor can be changed as appropriate. Forexample, to improve the aperture ratio, a structure where thesemiconductor film is in direct contact with the capacitor line withoutthe conductive film interposed therebetween can be employed. Specificexamples of the structure will be described with reference to FIG. 6 andFIG. 7. Here, only a capacitor 145 different from the capacitor 105described with reference to FIG. 2 and FIG. 3 will be described. FIG. 6is a top view of a pixel 141, and FIG. 7 is a cross-sectional view takenalong dashed-dotted lines A1-A2 and B1-B2 in FIG. 6.

In the pixel 141, the semiconductor film 119 functioning as oneelectrode of the capacitor 145 is in direct contact with the capacitorline 115 through an opening 143. Unlike in the capacitor 105 in FIG. 3,the semiconductor film 119 is in direct contact with the capacitor line115 without the conductive film 125 interposed therebetween and theconductive film 125 serving as a light-blocking film is not formed, sothat a higher aperture ratio of the pixel 141 can be achieved. To obtainthe above structure, an opening exposing the capacitor line 115 isformed before the semiconductor films 111 and 119 are formed in FIG. 4A.

Although the opening 143 is formed only over the capacitor line 115 inFIG. 7, an opening may be formed so as to expose part of the capacitorline 115 and part of the substrate 102 and the semiconductor film 119may be formed over the capacitor line 115 and the substrate 102 asillustrated in FIG. 8, in order to increase the area where thesemiconductor film 119 is in contact with the capacitor line 115. Toobtain the above structure, an opening exposing part of the capacitorline 115 and part of the substrate 102 is formed before thesemiconductor films 111 and 119 are formed in FIG. 4A, so that theaperture ratio can be improved and a capacitor 146 can be easily made tobe in a conductive state.

Modification Example 2

In the semiconductor device of one embodiment of the present invention,the conductive film which electrically connects the capacitor line andthe semiconductor film serving as one electrode of the capacitor can bechanged as appropriate. For example, to reduce contact resistancebetween the semiconductor film and the conductive film, the conductivefilm can be provided in contact with the semiconductor film along theouter periphery thereof. Specific examples of the structure will bedescribed with reference to FIG. 9 and FIGS. 10A and 10B. Here, only aconductive film 167 different from the conductive film 125 describedwith reference to FIG. 2 and FIG. 3 will be described. FIG. 9 is a topview of a pixel 161, FIG. 10A is a cross-sectional view taken alongdashed-dotted lines A1-A2 and B1-B2 in FIG. 9, and FIG. 10B is across-sectional view taken along dashed-dotted line D1-D2 in FIG. 9.

In the pixel 161, the conductive film 167 is in contact with thesemiconductor film 119 along the outer periphery thereof and is incontact with the capacitor line 115 through the opening 123 (see FIG.9). The conductive film 167 is formed in the same formation process asthe signal line 109 including the source electrode of the transistor 103and the conductive film 113 including the drain electrode of thetransistor 103 and thus may have a light-blocking property; for thisreason, the conductive film 167 is preferably formed into a loop shape.The structure of the pixel 161 in FIG. 9 is similar to that in FIG. 2,except for the conductive film 167.

As illustrated in FIGS. 10A and 10B, in the pixel 161, the conductivefilm 167 is provided so as to cover an end portion of the semiconductorfilm 119 of a capacitor 165 and be along the end portion.

In the structure illustrated in FIG. 9 and FIGS. 10A and 10B, theconductive film 167 is formed into a loop shape when seen from above;however, a portion of the conductive film 167 which is in contact withthe semiconductor film 119 does not have to be entirely electricallyconnected to the capacitor line 115. In other words, a conductive filmformed in the same formation process as the conductive film 167 may beprovided in contact with the semiconductor film 119 so as to be separatefrom the conductive film 167.

Modification Example 3

In the semiconductor device of one embodiment of the present invention,the structures of the semiconductor film included in the capacitor andthe capacitor line can be changed as appropriate. Specific examples ofthe structures will be described with reference to FIG. 11 and FIG. 12.Here, only a semiconductor film 177 and a capacitor line 175 differentfrom the semiconductor film 119 and the capacitor line 115 describedwith reference to FIG. 2 and FIG. 3 will be described. FIG. 11 is a topview of a pixel 171 where the capacitor line 175 is provided so as toextend in the direction parallel with the signal line 109. The signalline 109 and the capacitor line 175 are electrically connected to thesignal line driver circuit 106 (see FIG. 1A).

A capacitor 173 is connected to the capacitor line 175 provided so as toextend in the direction parallel with the signal line 109. The capacitor173 includes the semiconductor film 177 including an oxide semiconductorand formed similarly to the semiconductor film 111, the pixel electrode121, and an insulating film (not illustrated in FIG. 11) which is formedas a dielectric film over the transistor 103. The semiconductor film177, the pixel electrode 121, and the dielectric film transmit light;accordingly, the capacitor 173 transmits light.

Next, FIG. 12 is a cross-sectional view taken along dashed-dotted linesA1-A2 and B1-B2 in FIG. 11.

In the capacitor 173, the semiconductor film 177 formed in a mannersimilar to that of the semiconductor film 111 serves as one of a pair ofelectrodes, the pixel electrode 121 serves as the other of the pair ofelectrodes, and the insulating film 129, the insulating film 131, andthe insulating film 132 serve as a dielectric film provided between thepair of electrodes.

The capacitor line 175 can be formed concurrently with the signal line109 and the conductive film 113. When the capacitor line 175 is providedin contact with the semiconductor film 177, the area where thesemiconductor film 177 and the capacitor line 175 are in contact witheach other can be increased.

The pixel 171 illustrated in FIG. 11 has a shape with a side parallelwith the signal line 109 is longer than a side parallel with the scanline 107; however, like a pixel 172 illustrated in FIG. 13, the pixel171 may have a shape with a side parallel with the scan line 107 islonger than a side parallel with the signal line 109, and a capacitorline 176 may be provided so as to extend in the direction parallel withthe signal line 109. The signal line 109 and the capacitor line 176 areelectrically connected to the signal line driver circuit 106 (see FIG.1A).

A capacitor 174 is connected to the capacitor line 176 provided so as toextend in the direction parallel with the signal line 109. The capacitor174 includes a semiconductor film 178 including an oxide semiconductorand formed similarly to the semiconductor film 111, the pixel electrode121, and an insulating film (not illustrated in FIG. 13) which is formedover the transistor 103, as a dielectric film. The semiconductor film178, the pixel electrode 121, and the dielectric film transmit light;accordingly, the capacitor 174 transmits light.

Next, FIG. 14 is a cross-sectional view taken along dashed-dotted linesA1-A2 and B1-B2 in FIG. 13.

In the capacitor 174, the semiconductor film 178 formed in a mannersimilar to that of the semiconductor film 111 serves as one of a pair ofelectrodes, the pixel electrode 121 serves as the other of the pair ofelectrodes, and the insulating film 129, the insulating film 131, andthe insulating film 132 serve as a dielectric film provided between thepair of electrodes.

The capacitor line 176 can be formed concurrently with the signal line109 and the conductive film 113. When the capacitor line 176 is providedin contact with the semiconductor film 178, the area where thesemiconductor film 178 and the capacitor line 176 are in contact witheach other can be increased. The pixel 172 has a shape with a sideparallel with the signal line 109 is shorter than a side parallel withthe scan line 107; thus, the area where the pixel electrode 121 overlapswith the capacitor line 176 can be small as compared with the case ofthe pixel 171 illustrated in FIG. 11, resulting in a higher apertureratio.

Modification Example 4

In the semiconductor device of one embodiment of the present invention,one electrode of the capacitor and the capacitor line can be formedusing a semiconductor film (specifically, an oxide semiconductor film).A specific example will be described with reference to FIG. 37. Here,only a semiconductor film 198 different from the semiconductor film 119and the capacitor line 115 described with reference to FIG. 2 will bedescribed. FIG. 37 is a top view of a pixel 196 where the semiconductorfilm 198 serving as one electrode of a capacitor 197 and the capacitorline is provided in the pixel 196. The semiconductor film 198 has aregion which extends in the direction parallel with the signal line 109and the region functions as the capacitor line. In the semiconductorfilm 198, a region which overlaps with the pixel electrode 121 functionsas one electrode of the capacitor 197. The semiconductor film 198 can beformed concurrently with the semiconductor film 111 of the transistor103 provided in the pixel 196.

In the case where a continuous oxide semiconductor film is provided asthe semiconductor film 198 for the pixels 196 in one row, thesemiconductor film 198 overlaps with the scan lines 107. For thisreason, the semiconductor film 198 does not function as the capacitorline and one electrode of the capacitor 197 due to an effect of a changein the potential of the scan line 107 in some cases. Thus, thesemiconductor films 198 are provided for the respective pixels 196 so asto be separate from each other as illustrated in FIG. 37. Further, thesemiconductor films 198 provided so as to be separate from each otherare preferably electrically connected to each other using a conductivefilm 199 which can be formed concurrently with the signal line 109 andthe conductive film 113. With the above structure, a portion of thesemiconductor film 198 which is not connected to the conductive film 199overlaps with the pixel electrode 121, whereby the resistance of thesemiconductor film 198 in the region can be low and thus thesemiconductor film 198 functions as the capacitor line and one electrodeof the capacitor 197.

Although not illustrated, one oxide semiconductor film can be providedas the semiconductor film 198 for the pixels 196 so as to overlap thescan lines 107 in the case where a portion of the semiconductor film 198which overlaps with the scan line 107 is not influenced by a change inthe potential of the scan line 107. In other words, a continuous oxidesemiconductor film can be provided as the semiconductor film 198 for thepixels 196 in one row.

In FIG. 37, a portion of the semiconductor film 198 which functions asthe capacitor line extends in the direction parallel with the signalline 109; however, the region which functions as the capacitor line mayextend in the direction parallel with the scan line 107. In the casewhere the portion of the semiconductor film 198 which functions as thecapacitor line extends in the direction parallel with the scan line 107,it is necessary that the semiconductor film 111 and the semiconductorfilm 198 be electrically insulated from the signal line 109 and theconductive film 113 by providing an insulating film between thesemiconductor film 111 and the semiconductor film 198, and the signalline 109 and the conductive film 113, in the transistor 103 and thecapacitor 197.

According to the above description, when a light-transmitting oxidesemiconductor film is provided for one electrode of a capacitor providedin a pixel and a capacitor line as in the pixel 196, the pixel can havea higher aperture ratio.

Modification Example 5

In the semiconductor device of one embodiment of the present invention,the structure of the capacitor line can be changed as appropriate. Thisstructure will be described with reference to FIG. 35. In FIG. 35,unlike the capacitor line 115 described with reference to FIG. 2, acapacitor line is located between adjacent two pixels.

FIG. 35 illustrates a structure where the capacitor line is providedbetween the pixels adjacent to each other in the direction in which asignal line 409 extends. FIG. 48 illustrates a structure where acapacitor line is provided between pixels adjacent to each other in thedirection in which a scan line 437 extends.

FIG. 35 is a top view of pixels 401_1 and 401_2 adjacent to each otherin the direction in which the signal line 409 extends.

Scan lines 407_1 and 407_2 are provided so as to extend in parallel witheach other in the direction perpendicular to or substantiallyperpendicular to the signal line 409. A capacitor line 415 is providedbetween the scan lines 407_1 and 407_2 so as to be parallel with thescan lines 407_1 and 407_2. The capacitor line 415 is connected to acapacitor 405_1 provided in the pixel 401_1 and a capacitor 405_2provided in the pixel 401_2. Top surface shape and the positions ofcomponents of the pixel 401_1 and those of the pixel 401_2 are symmetricwith respect to the capacitor line 415.

The pixel 401_1 is provided with a transistor 403_1, a pixel electrode421_1 connected to the transistor 403_1, and the capacitor 405_1.

The transistor 403_1 is provided in a region where the scan line 407_1and the signal line 409 cross each other. The transistor 403_1 includesat least a semiconductor film 411_1 including a channel formationregion, a gate electrode, a gate insulating film (not illustrated inFIG. 35), a source electrode, and a drain electrode. A portion of thescan line 407_1 which overlaps with the semiconductor film 411_1functions as the gate electrode of the transistor 403_1. A portion ofthe signal line 409 which overlaps with the semiconductor film 411_1functions as the source electrode of the transistor 403_1. A portion ofthe conductive film 413_1 which overlaps with the semiconductor film411_1 functions as the drain electrode of the transistor 403_1. Theconductive film 413_1 and the pixel electrode 421_1 are connected toeach other through an opening 417_1.

The capacitor 405_1 is electrically connected to the capacitor line 415through the conductive film 425 provided in and over the opening 423.The capacitor 405_1 includes a semiconductor film 419_1 including anoxide semiconductor, the pixel electrode 421_1, and an insulating film(not illustrated in FIG. 35) which is formed as a dielectric film overthe transistor 403_1. The semiconductor film 419_1, the pixel electrode421_1, and the dielectric film transmit light; accordingly, thecapacitor 405_1 transmits light.

The pixel 401_2 is provided with a transistor 403_2, a pixel electrode421_2 connected to the transistor 403_2, and a capacitor 405_2.

The transistor 403_2 is provided in a region where the scan line 407_2and the signal line 409 cross each other. The transistor 403_2 includesat least a semiconductor film 411_2 including a channel formationregion, a gate electrode, a gate insulating film (not illustrated inFIG. 35), a source electrode, and a drain electrode. A portion of thescan line 407_2 which overlaps with the semiconductor film 411_2functions as the gate electrode of the transistor 403_2. A portion ofthe signal line 409 which overlaps with the semiconductor film 411_2functions as the source electrode of the transistor 403_2. A portion ofthe conductive film 413_2 which overlaps with the semiconductor film411_2 functions as the drain electrode of the transistor 403_2. Theconductive film 413_2 and the pixel electrode 421_2 are connected toeach other through an opening 417_2.

The capacitor 405_2 is electrically connected to the capacitor line 415through the conductive film 425 provided in and over the opening 423similarly to the capacitor 405_1. The capacitor 405_2 includes asemiconductor film 419_2 including an oxide semiconductor, the pixelelectrode 421_2, and an insulating film (not illustrated in FIG. 35)which is formed over the transistor 403_2 and serves as a dielectricfilm. The semiconductor film 419_2, the pixel electrode 421_2, and thedielectric film transmit light; accordingly, the capacitor 405_2transmits light.

Cross-sectional structures of the transistors 403_1 and 403_2 and thecapacitors 405_1 and 405_2 are similar to those of the transistor 103and the capacitor 105 illustrated in FIG. 3 and thus descriptionsthereof are omitted here.

Although the capacitor line is provided between the pixels adjacent toeach other in the direction in which the signal line 409 extends in FIG.35, the capacitor line may be provided between the pixels adjacent toeach other in the direction in which the scan line 437 extends as inFIG. 48.

FIG. 48 is a top view of pixels 431_1 and 431_2 adjacent to each otherin the direction in which the scan line 437 extends.

Signal lines 439_1 and 439_2 are provided so as to extend in parallelwith each other in the direction perpendicular to or substantiallyperpendicular to the scan line 437. A capacitor line 445 is providedbetween the signal lines 439_1 and 439_2 so as to be parallel with thesignal lines 439_1 and 439_2. The capacitor line 445 is connected to acapacitor 435_1 provided in the pixel 431_1 and a capacitor 435_2provided in the pixel 431_2. Top surface shape and the positions ofcomponents of the pixel 431_1 and those of the pixel 431_2 are symmetricwith respect to the capacitor line 445.

The pixel 431_1 is provided with a transistor 433_1, the pixel electrode451_1 connected to the transistor 433_1, and the capacitor 435_1.

The transistor 433_1 is provided in a region where the scan line 437 andthe signal line 439_1 cross each other. The transistor 433_1 includes atleast a semiconductor film 441_1 including a channel formation region, agate electrode, a gate insulating film (not illustrated in FIG. 48), asource electrode, and a drain electrode. A portion of the scan line 437which overlaps with the semiconductor film 441_1 functions as the gateelectrode of the transistor 433_1. A portion of the signal line 439_1which overlaps with the semiconductor film 441_1 functions as the sourceelectrode of the transistor 433_1. A portion of the conductive film443_1 which overlaps with the semiconductor film 441_1 functions as thedrain electrode of the transistor 433_1. The conductive film 443_1 andthe pixel electrode 421_1 are connected to each other through an opening447_1.

The capacitor 435_1 is connected to the capacitor line 445. Thecapacitor 435_1 includes the semiconductor film 449_1 including an oxidesemiconductor, the pixel electrode 451_1, and an insulating film (notillustrated in FIG. 48) which is formed as a dielectric film over thetransistor 433_1. The semiconductor film 449_1, the pixel electrode451_1, and the dielectric film transmit light; accordingly, thecapacitor 435_1 transmits light.

The pixel 431_2 is provided with a transistor 433_2, a pixel electrode451_2 connected to the transistor 433_2, and a capacitor 435_2.

The transistor 433_2 is provided in a region where the scan line 437 andthe signal line 439_2 cross each other. The transistor 433_2 includes atleast a semiconductor film 441_2 including a channel formation region, agate electrode, a gate insulating film (not illustrated in FIG. 48), asource electrode, and a drain electrode. A portion of the scan line 437which overlaps with the semiconductor film 441_2 functions as the gateelectrode of the transistor 433_2. A portion of the signal line 439_2which overlaps with the semiconductor film 441_2 functions as the sourceelectrode of the transistor 433_2. A portion of the conductive film443_2 which overlaps with the semiconductor film 441_2 functions as thedrain electrode of the transistor 433_2. The conductive film 443_2 andthe pixel electrode 451_2 are connected to each other through an opening447_2.

The capacitor 435_2 is electrically connected to the capacitor line 445similarly to the capacitor 435_1. The capacitor 435_2 includes thesemiconductor film 449_2 including an oxide semiconductor, the pixelelectrode 451_2, and an insulating film (not illustrated in FIG. 48)which is formed as a dielectric film over the transistor 433_2. Thesemiconductor film 449_2, the pixel electrode 451_2, and the dielectricfilm transmit light; accordingly, the capacitor 435_2 transmits light.

Cross-sectional structures of the transistors 433_1 and 433_2 and thecapacitors 435_1 and 435_2 are similar to those of the transistor 103and the capacitor 105 illustrated in FIG. 3 and thus descriptionsthereof are omitted here.

In a structure seen from above, a capacitor line is provided betweenadjacent two pixels so that capacitors included in the pixels and thecapacitor line are connected, whereby the number of capacitor lines canbe reduced. As a result, the aperture ratio of the pixel can be high ascompared with the case of a structure where each pixel is provided witha capacitor line.

Modification Example 6

To reduce parasitic capacitance generated between the pixel electrode121 and the conductive film 113 and parasitic capacitance generatedbetween the pixel electrode 121 and the conductive film 125 in thepixels 101, 141, 151, 161, 171, 172, 401_1, and 401_2, an organicinsulating film 134 can be provided in a region where the parasiticcapacitance is generated as illustrated in a cross-sectional view inFIG. 15. The structure in FIG. 15 is the same as that in FIG. 3 exceptfor the organic insulating film 134. Here, only the organic insulatingfilm 134 not included in the structure in FIG. 3 will be described.

For the organic insulating film 134, a photosensitive organic resin or anon-photosensitive organic resin can be used; for example, an acrylicresin, a benzocyclobutene resin, an epoxy resin, a siloxane resin, orthe like can be used. Alternatively, polyamide can be used for theorganic insulating film 134.

The organic insulating film 134 can be formed in such manner that anorganic resin film is formed using any of the materials listed above andprocessed. When a photosensitive organic resin is used for the organicinsulating film 134, a resist mask is unnecessary in formation of theorganic insulating film 134 and thus a process can be simplified. Notethat a formation method of the organic insulating film is notparticularly limited and can be selected as appropriate in accordancewith a material which is used. For example, spin coating, dipping, spraycoating, a droplet discharge method (e.g., an ink-jet method), screenprinting, offset printing, or the like can be employed.

In general, an organic resin contains much hydrogen and water; thus,when an organic resin is provided over the transistor 103 (inparticular, the semiconductor film 111), hydrogen and water contained inthe organic resin diffuses into the transistor 103 (in particular, thesemiconductor film 111) and might degrade the electrical characteristicsof the transistor 103. For this reason, it is preferable that theorganic insulating film 134 be not provided at least over a portion ofthe insulating film 132 which overlaps with the semiconductor film 111.In other words, it is preferable that a portion of the organic resinfilm which is over a region overlapping at least the semiconductor film111 be removed.

FIG. 16 is a top view of the pixel 101 shown in FIG. 15. Thecross-sectional view in FIG. 15 corresponds to cross sections takenalong dashed-dotted lines A1-A2, B1-B2, and C1-C2 in FIG. 16. In FIG.16, the organic insulating film 134 is not illustrated forsimplification; however, a region indicated by dashed-two dotted linesis a region where the organic insulating film 134 is not provided.

Modification Example 7

In the semiconductor device of one embodiment of the present invention,the shape of a transistor provided in a pixel is not limited to theshape of the transistor illustrated in FIG. 2 and FIG. 3 and can bechanged as appropriate. For example, as illustrated in FIG. 17, in thepixel 151, a transistor 169 may be different from the transistor 103 inthat a source electrode included in the signal line 109 has a U shape(or a C shape, a square-bracket-like shape, or a horseshoe shape) whichpartly surrounds the conductive film 113 including a drain electrode.With such a shape, a sufficient channel width can be ensured even whenthe area of the transistor is small, and accordingly, the amount ofdrain current flowing at the time of conduction of the transistor (alsoreferred to as an on-state current) can be increased. The structure ofthe pixel 151 in FIG. 17 is similar to that in FIG. 2, except for thetransistor 169.

Modification Example 8

Although in the pixels 101, 141, 151, 161, 171, 172, 401_1, and 401_2described above, a transistor where the oxide semiconductor film isprovided between the signal line 109 including the gate insulating filmand the source electrode and the conductive film 113 including the drainelectrode is used, instead of the transistor, a transistor 190 where asemiconductor film 195 is provided between the insulating film 129, anda signal line 191 including a source electrode and a conductive film 193including a drain electrode as illustrated in FIG. 18 can be used. Thestructure in FIG. 18 is the same as that in FIG. 3 except for theposition of the semiconductor film 195.

In the transistor 190 illustrated in FIG. 18, the signal line 191 andthe conductive film 193 are formed and then the semiconductor film 195is formed. Thus, a surface of the semiconductor film 195 is not exposedto an etchant or an etching gas used in a formation process of thesignal line 191 and the conductive film 193, so that impurities betweenthe semiconductor film 195 and the insulating film 129 can be reduced.Accordingly, a leakage current flowing between the source electrode andthe drain electrode of the transistor 190 can be reduced.

Modification Example 9

Although in the pixels 101, 141, 151, 161, 171, 172, 401_1, and 401_2described above, a channel-etched transistor is used as the transistor,instead of the transistor, a channel protective transistor 183 can beused as illustrated in FIG. 19. The structure in FIG. 19 is the same asthat in FIG. 3 except that a channel protective film 182 is providedbetween the semiconductor film 111, and the signal line 109 includingthe source electrode and the conductive film 113 including the drainelectrode.

In the transistor 183 in FIG. 19, the channel protective film 182 isformed over the semiconductor film 111 and then the signal line 109 andthe conductive film 113 are formed. The channel protective film 182 canbe formed using the material of the insulating film 129 formed over thetransistor 103, in which case it is not necessary to additionallyprovide an insulating film corresponding to the insulating film 129formed over the transistor 103 in the transistor 183. Further, when thechannel protective film 182 is provided, a surface of the semiconductorfilm 111 is not exposed to an etchant or an etching gas used in aformation process of the signal line 109 and the conductive film 113, sothat impurities between the semiconductor film 111 and the channelprotective film 182 can be reduced. Accordingly, a leakage currentflowing between the source electrode and the drain electrode of thetransistor 183 can be reduced.

Modification Example 10

Although in the pixels 101, 141, 151, 161, 171, 172, 401_1, and 401_2described above, a transistor having one gate electrode is used, insteadof the transistor, a transistor 185 having two gate electrodes facingeach other with the semiconductor film 111 interposed therebetween asillustrated in FIG. 36A can be used.

The transistor 185 is different from the transistors 103, 169, and 190described in this embodiment in that a conductive film 187 is providedover the insulating film 132 over the transistor. The conductive film187 overlaps with at least a channel formation region of thesemiconductor film 111. It is preferable that the conductive film 187 beprovided in a position overlapping the channel formation region of thesemiconductor film 111 so that the potential of the conductive film 187is equal to the minimum potential of a video signal input to the signalline 109. In that case, a current flowing between the source electrodeand the drain electrode in the surface portion of the semiconductor film111 facing the conductive film 187 can be controlled, and variations inthe electrical characteristics of the transistors can be reduced.Further, when the conductive film 187 is provided, an influence of achange in ambient electric field on the semiconductor film 111 can bereduced, leading to an improvement in reliability of the transistor.

The conductive film 187 can be formed using a material and a methodsimilar to those of the scan line 107, the signal line 109, the pixelelectrode 121, or the like.

The conductive film 187 illustrated in FIG. 36A partly overlaps withsource and drain electrodes; however, a structure where a conductivefilm 687 overlaps with a gate electrode 307 and does not overlap eithera source electrode 309 or a drain electrode 613 as in a transistor 685illustrated in FIG. 36B may be employed.

As described above, the use of the semiconductor film formed in the sameformation step as the semiconductor film included in the transistor, forone electrode of the capacitor, allows fabrication of a semiconductordevice including the capacitor whose charge capacity is increased whileimproving the aperture ratio. As a result, the semiconductor device canhave excellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in the oxidesemiconductor film, which is a semiconductor film included in thetransistor, are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments and example.

Embodiment 2

In this embodiment, a semiconductor device of one embodiment of thepresent invention which has a structure different from that in the aboveembodiment will be described with reference to drawings. A semiconductordevice of one embodiment of the present invention will be describedtaking a liquid crystal display device as an example in this embodiment.In the semiconductor device described in this embodiment, the structureof a capacitor is different from that of the capacitor in the aboveembodiment. The above embodiment can be referred to for components inthe semiconductor device in this embodiment which are similar to thoseof the semiconductor device in the above embodiment.

<Structure of Semiconductor Device>

FIG. 20 is a top view of a pixel 201 in this embodiment. In the pixel201 in FIG. 20, an insulating film 229 (not illustrated) and aninsulating film 231 (not illustrated) are not provided in a regionindicated by dashed-two dotted lines. Thus, a capacitor 205 in the pixel201 in FIG. 20 includes the semiconductor film 119 serving as oneelectrode, a pixel electrode 221 serving as the other electrode, and aninsulating film 232 (not illustrated) serving as a dielectric film.

Next, FIG. 21 is a cross-sectional view taken along dashed-dotted linesA1-A2 and B1-B2 in FIG. 20.

A cross-sectional structure of the pixel 201 in this embodiment is asfollows. The scan line 107 including a gate electrode of the transistor103 and the capacitor line 115 over the same surface as the scan line107 are provided over the substrate 102. A gate insulating film 127 isprovided over the scan line 107 and the capacitor line 115. Thesemiconductor film 111 is provided over a portion of the gate insulatingfilm 127 which overlaps with the scan line 107, and the semiconductorfilm 119 is provided over the gate insulating film 127. The signal line109 including a source electrode of the transistor 103 and theconductive film 113 including a drain electrode of the transistor 103are provided over the semiconductor film 111 and the gate insulatingfilm 127. The opening 123 reaching the capacitor line 115 is formed inthe gate insulating film 127, and the conductive film 125 is provided inand over the opening 123 and over the gate insulating film 127 and thesemiconductor film 119. The insulating film 229, the insulating film231, and the insulating film 232 functioning as protective insulatingfilms of the transistor 103 are provided over the gate insulating film127, the signal line 109, the semiconductor film 111, the conductivefilm 113, the conductive film 125, and the semiconductor film 119. Theinsulating film 232 is provided at least over a portion of thesemiconductor film 119 which serves as the capacitor 205. The opening117 reaching the conductive film 113 is formed in the insulating film229, the insulating film 231, and the insulating film 232, and the pixelelectrode 221 is provided in and over the opening 117 and over theinsulating film 232. Note that a base insulating film may be providedbetween the substrate 102, and the scan line 107, the capacitor line115, and the gate insulating film 127.

The insulating film 229 is similar to the insulating film 129 describedin Embodiment 1. The insulating film 231 is similar to the insulatingfilm 131 described in Embodiment 1. The insulating film 232 is similarto the insulating film 132 described in Embodiment 1. The pixelelectrode 221 is similar to the pixel electrode 121 described inEmbodiment 1.

When the insulating film 232 serves as a dielectric film between thesemiconductor film 119 serving as one electrode and the pixel electrode221 serving as the other electrode as in the capacitor 205 in thisembodiment, the thickness of the dielectric film can be thinner thanthat of the dielectric film of the capacitor 105 in Embodiment 1. Thus,the capacitor 205 in this embodiment can have larger charge capacitythan the capacitor 205 in Embodiment 1.

The insulating film 232 is preferably a nitride insulating filmsimilarly to the insulating film 132 in Embodiment 1. The insulatingfilm 232 is in contact with the semiconductor film 119, so that nitrogenor hydrogen contained in the nitride insulating film can enter thesemiconductor film 119 and thus the semiconductor film 119 can be ann-type semiconductor film and have higher conductivity. Further, whenthe insulating film 232 is formed using a nitride insulating film and issubjected to heat treatment while it is in contact with thesemiconductor film 119, nitrogen or hydrogen contained in the nitrideinsulating film can be released to the semiconductor film 119.

The semiconductor film 119 has a region with higher conductivity thanthat of the semiconductor film 111. With this structure, a portion ofthe semiconductor film 119 which is in contact with the insulating film232 is n-type and has higher conductivity than a portion of thesemiconductor film 111 which is in contact with the insulating film 229.

Note that in FIG. 20, an edge of a region (indicated by dashed-twodotted lines) where the insulating film 229 (not illustrated) and theinsulating film 231 (not illustrated) are not provided is on the outerside than the semiconductor film 119; however, an edge of a region(indicated by dashed-two dotted lines) where an insulating film 279 (notillustrated) and the insulating film 281 (not illustrated) are notprovided may be over the semiconductor film 119 as illustrated in FIG.46.

FIG. 47 is a cross-sectional view taken along dashed-dotted lines A1-A2and B1-B2 in FIG. 46.

In FIG. 47, the insulating film 279, the insulating film 281, and aninsulating film 282 functioning as protective insulating films of thetransistor 103 are provided over the gate insulating film 127, thesignal line 109, the semiconductor film 111, the conductive film 113,the conductive film 125, and the semiconductor film 119. Edges of theinsulating film 279 and the insulating film 281 overlap thesemiconductor film 119. A capacitor 255 includes the semiconductor film119, the insulating film 282, and a pixel electrode 271. The insulatingfilm 279, the insulating film 281, and the insulating film 282 aresimilar to the insulating film 129, the insulating film 131, and theinsulating film 132 described in Embodiment 1. The pixel electrode 271is similar to the pixel electrode 121 described in Embodiment 1. Asillustrated in FIG. 47, edges of the insulating film 279 and theinsulating film 281 overlap the semiconductor film 119, so that the gateinsulating film 127 can be prevented from being excessively etched inetching of the insulating film 279 and the insulating film 281.

In an operation method of the capacitor 205 in the semiconductor deviceof this embodiment, the potential of the semiconductor film 119 (inother words, the potential of the capacitor line 115) is constantlylower than the potential of the pixel electrode 121 by greater than orequal to the threshold voltage (Vth) of the capacitor 205 (MOScapacitor) in a period when the capacitor 205 is operated, as in theoperation method of the capacitor 105 in Embodiment 1. However, in thecapacitor 205, the semiconductor film 119 serving as one electrode isn-type and has high conductivity, so that the threshold voltage (Vth) isshifted in the negative direction as shown by a dashed line in FIG. 38.The potential of the semiconductor film 119 (in other words, thepotential of the capacitor line 115) can be raised in accordance withthe shift amount of the threshold voltage (Vth) of the capacitor 205 inthe negative direction, from the lowest potential of the pixel electrode121. Therefore, in the case where the threshold voltage of the capacitor205 is a larger negative value, the potential of the capacitor line 115can be higher than the potential of the pixel electrode 121 as in FIG.39B.

When the semiconductor film 119 serving as one electrode of thecapacitor 205 is n-type and has high conductivity as in this embodiment,the threshold voltage can be shifted in the negative direction, so thatthe range of the potential needed for operating the capacitor 205 can bemade large as compared with the case of the capacitor 105 inEmbodiment 1. Thus, in this embodiment, the capacitor 205 can beconstantly operated with stability in an operation period of thecapacitor 205, which is preferable.

Further, since the semiconductor film 119 included in the capacitor 205is n-type and has high conductivity, enough charge capacity can beobtained even when the plane area of the capacitor 205 is reduced. Anoxide semiconductor included in the semiconductor film 119 transmits 80%to 90% of light; thus, when the area of the semiconductor film 119 isreduced and a region where the semiconductor film 119 is not formed isprovided in the pixel, the transmissivity with respect to light emittedfrom a light source such as a backlight can be increased.

<Fabrication Method of Semiconductor Device>

Next, a fabrication method of the semiconductor device of thisembodiment will be described with reference to FIGS. 22A and 22B andFIGS. 23A and 23B.

First, the scan line 107 and the capacitor line 115 are formed over thesubstrate 102. An insulating film which is to be processed into the gateinsulating film 127 is formed over the substrate 102, the scan line 107,and the capacitor line. The semiconductor film 111 and the semiconductorfilm 119 are formed over the insulating film. The opening 123 reachingthe capacitor line 115 is formed in the insulating film to form the gateinsulating film 127 and then the signal line 109, the conductive film113, and the semiconductor film 125 are formed. The insulating film 128is formed over the gate insulating film 127, the signal line 109, theconductive film 113, the conductive film 125, and the semiconductor film119. The insulating film 130 is formed over the insulating film 128 (seeFIG. 22A). The above steps can be performed with reference to Embodiment1.

Then, a mask is formed over a portion of the insulating film 130 whichoverlaps with at least the semiconductor film 119. Processing isperformed to form an insulating film 228 and an insulating film 230 withthe use of the mask and expose the semiconductor film 119. An insulatingfilm 233 is formed over the exposed region and the insulating film 130(see FIG. 22B). As the mask, a resist mask formed through aphotolithography process can be used, and the processing can beperformed by one of or both dry etching and wet etching. The insulatingfilm 233 is similar to the insulating film 133 described inEmbodiment 1. Note that heat treatment may be performed while theinsulating film 233 is in contact with the semiconductor film 119, forexample, after formation of the insulating film 233. The above steps canalso be performed with reference to Embodiment 1.

Then, the opening 117 reaching the conductive film 113 is formed in theinsulating film 228, the insulating film 230, and the insulating film233 to form the insulating film 229, the insulating film 231, and theinsulating film 232 (see FIG. 23A). The pixel electrode 221 in contactwith the conductive film 113 through the opening 117 is formed (see FIG.23B). The above steps can also be performed with reference to Embodiment1.

Through the above steps, the semiconductor device of this embodiment canbe fabricated.

Modification Example

In the semiconductor device of one embodiment of the present invention,the structure of the capacitor can be changed as appropriate. A specificexample of the structure will be described with reference to FIG. 24.Here, only a capacitor 245 different from the capacitor 105 describedwith reference to FIG. 2 and FIG. 3 will be described.

In order that the semiconductor film 119 be n-type and have higherconductivity, the gate insulating film 227 has a layered structure ofthe insulating film 225 formed of a nitride insulating film and theinsulating film 226 formed of an oxide insulating film and only theinsulating film 225 is provided in a region where at least thesemiconductor film 119 is provided. With such a structure, the nitrideinsulating film forming the insulating film 225 is in contact with thebottom surface of the semiconductor film 119, so that the semiconductorfilm 119 can be n-type and have higher conductivity (see FIG. 24). Inthis case, a dielectric film of the capacitor 245 is the insulating film129, the insulating film 131, and the insulating film 132. As theinsulating film 225 and the insulating film 226, insulating films whichcan be used as the gate insulating film 127 can be used as appropriate,and the insulating film 225 may be formed using an insulating filmsimilar to the insulating film 132. Further, to obtain this structure,the insulating film 226 is processed as appropriate with reference toEmbodiment 1. The structure illustrated in FIG. 24 can prevent areduction in the thickness of the semiconductor film 119 due to etchingof the insulating film 129 and the insulating film 131, so that theyield is increased as compared with the semiconductor device illustratedin FIG. 21.

In the structure illustrated in FIG. 24, the top surface of thesemiconductor film 119 may be in contact with the insulating film 132.That is, portions of the insulating film 129 and the insulating film 131in FIG. 24 which are in contact with the semiconductor film 119 may beremoved. In that case, a dielectric film of the capacitor 245 is theinsulating film 132. When the top and bottom surfaces of thesemiconductor film 119 are in contact with the nitride insulating film,the semiconductor film 119 can be n-type and have higher conductivitymore efficiently and sufficiently than the semiconductor film 119 whichis in contact with only one of surfaces of the nitride insulating film.

As described above, the use of the semiconductor film formed through thesame formation step as the semiconductor film included in thetransistor, for one electrode of the capacitor, allows fabrication of asemiconductor device including the capacitor whose charge capacity isincreased while improving the aperture ratio to typically 55% or more,preferably 60% or more. As a result, the semiconductor device can haveexcellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in the oxidesemiconductor film, which is a semiconductor film included in thetransistor, are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and modificationexamples thereof described in the other embodiments and example.

Embodiment 3

In this embodiment, a semiconductor device of one embodiment of thepresent invention which has a structure different from that in the aboveembodiment will be described with reference to the drawings. Asemiconductor device of one embodiment of the present invention will bedescribed taking a liquid crystal display device as an example in thisembodiment. In the semiconductor device described in this embodiment, asemiconductor film included in a capacitor is different from that in thecapacitor in the above embodiment. The above embodiment can be referredto for components in the semiconductor device in this embodiment whichare similar to those of the semiconductor device in the aboveembodiment.

<Structure of Semiconductor Device>

Next, a specific example of the structure of the pixel 301 provided in apixel portion of the liquid crystal display device described in thisembodiment will be described. FIG. 25 is a top view of the pixel 301.The pixel 301 in FIG. 25 is provided with a capacitor 305 provided in aregion surrounded by the capacitor lines 115 and the signal lines 109 inthe pixel 301. The capacitor 305 is electrically connected to thecapacitor line 115 through the conductive film 125 provided in and overthe opening 123. The capacitor 305 includes a semiconductor film 319including an oxide semiconductor and having higher conductivity than thesemiconductor film 111, the pixel electrode 121, and an insulating film(not illustrated in FIG. 25) which is formed as a dielectric film overthe transistor 103. The semiconductor film 319, the pixel electrode 121,and the dielectric film transmit light; accordingly, the capacitor 305transmits light.

In the case where the semiconductor film 319 is an oxide semiconductorfilm, the conductivity of the oxide semiconductor film is greater thanor equal to 10 S/cm and less than or equal to 1000 S/cm, preferablygreater than or equal to 100 S/cm and less than or equal to 1000 S/cm.

As described above, the semiconductor film 319 transmits light. That isto say, the capacitor 305 can be formed large (in a large area) in thepixel 301. Therefore, the semiconductor device can have charge capacityincreased while improving the aperture ratio to typically 55% or more,preferably 60% or more. As a result, the semiconductor device can haveexcellent display quality. Further, since the semiconductor film 319included in the capacitor 305 is n-type and has high conductivity,enough charge capacity can be obtained even when the plane area of thecapacitor 305 is reduced. An oxide semiconductor included in thesemiconductor film 319 transmits 80 to 90% of light; thus, when the areaof the semiconductor film 319 is reduced and a region where thesemiconductor film 319 is not formed is provided in the pixel, thetransmissivity with respect to light emitted from a light source such asa backlight can be increased.

Next, FIG. 26 is a cross-sectional view taken along dashed-dotted linesA1-A2 and B1-B2 in FIG. 25.

A cross-sectional structure of the pixel 301 of a liquid crystal displaydevice is as follows. The scan line 107 including the gate electrode ofthe transistor 103 is provided over the substrate 102. The gateinsulating film 127 is provided over the scan line 107. Thesemiconductor film 111 is provided over a portion of the gate insulatingfilm 127 which overlaps with the scan line 107, and the semiconductorfilm 319 is provided over the gate insulating film 127. The signal line109 including the source electrode of the transistor 103 and theconductive film 113 including the drain electrode of the transistor 103are provided over the semiconductor film 111 and the gate insulatingfilm 127. Further, the capacitor line 115 is provided over the gateinsulating film 127 and the semiconductor film 319. The insulating film129, the insulating film 131, and the insulating film 132 functioning asprotective insulating films of the transistor 103 are provided over thegate insulating film 127, the signal line 109, the semiconductor film111, the conductive film 113, the semiconductor film 319, and thecapacitor line 115. The opening 117 reaching the conductive film 113 isformed in the insulating film 129, the insulating film 131, and theinsulating film 132, and the pixel electrode 121 is provided in theopening 117 and over the insulating film 132. Note that a baseinsulating film may be provided between the substrate 102, and the scanline 107 and the gate insulating film 127.

In the capacitor 105 in this example, the semiconductor film 319 whichis n-type and has higher conductivity than the semiconductor film 111serves as one of a pair of electrodes, the pixel electrode 121 serves asthe other of the pair of electrodes, and the insulating film 129, theinsulating film 131, and the insulating film 132 serve as a dielectricfilm provided between the pair of electrodes.

For the semiconductor film 319, an oxide semiconductor which can be usedfor the semiconductor film 111 can be used. The semiconductor film 319can be formed concurrently with the semiconductor film 111 and thuscontains a metal element of an oxide semiconductor included in thesemiconductor film 111. Further, the semiconductor film 319 preferablyhas higher conductivity than the semiconductor film 111 and thuspreferably contains an element (dopant) which increases theconductivity. Specifically, the semiconductor film 319 contains one ormore selected from boron, nitrogen, fluorine, aluminum, phosphorus,arsenic, indium, tin, antimony, and a rare gas element. Theconcentration of a dopant contained in the semiconductor film 319 ispreferably greater than or equal to 1×10¹⁹ atoms/cm³ and less than orequal to 1×10²² atoms/cm³, in which case the conductivity of thesemiconductor film 319 can be greater than or equal to 10 S/cm and lessthan or equal to 1000 S/cm, preferably greater than or equal to 100 S/cmand less than or equal to 1000 S/cm, so that the semiconductor film 319can sufficiently function as one electrode of the capacitor 305. Thesemiconductor film 319 has a region with higher conductivity than thatof the semiconductor film 111. With this structure, a portion of thesemiconductor film 319 which is in contact with the insulating film 132has higher conductivity than a portion of the semiconductor film 111which is in contact with the insulating film 129.

<Fabrication Method of Semiconductor Device>

Next, a fabrication method of the semiconductor device of thisembodiment will be described with reference to FIGS. 27A and 27B andFIGS. 28A and 28B.

First, the scan line 107 and the capacitor line 115 are formed over thesubstrate 102. An insulating film which is to be processed into the gateinsulating film 127 is formed over the substrate 102, the scan line 107,and the capacitor line. The semiconductor film 111 and the semiconductorfilm 119 are formed over the insulating film (see FIG. 27A). The abovesteps can be performed with reference to Embodiment 1.

After that, the semiconductor film 119 is doped with a dopant to formthe semiconductor film 319, the opening 123 reaching the capacitor line115 is formed in the insulating film 126 to form the gate insulatingfilm 127, and then the signal line 109 including the source electrode ofthe transistor 103, the conductive film 113 including the drainelectrode of the transistor 103, and the conductive film 125 whichelectrically connects the semiconductor film 319 and the capacitor line115 are formed (see FIG. 27B).

A method of doping the semiconductor film 119 with a dopant is asfollows: a mask is provided in a region except the semiconductor film119 and the semiconductor film 119 is doped with one or more dopantsselected from boron, nitrogen, fluorine, aluminum, phosphorus, arsenic,indium, tin, antimony, and a rare gas element by an ion implantationmethod, an ion doping method, or the like. Alternatively, thesemiconductor film 119 may be exposed to plasma containing the dopant todope the semiconductor film 119 with the dopant, instead of employing anion implantation method or an ion doping method. Note that heattreatment may be performed after the semiconductor film 119 is dopedwith the dopant. The heat treatment can be performed as appropriate withreference to the details of heat treatment for dehydration ordehydrogenation of the semiconductor film 111 and the semiconductor film119.

The step of doping with the dopant may be performed after formation ofthe signal line 109, the conductive film 113, and the conductive film125, in which case a portion of the semiconductor film 319 which is incontact with the signal line 109, the conductive film 113, and theconductive film 125 is not doped with the dopant.

Then, the insulating film 128 is formed over the gate insulating film127, the signal line 109, the semiconductor film 111, the conductivefilm 113, the conductive film 125, and the semiconductor film 319. Theinsulating film 130 is formed over the insulating film 128, and theinsulating film 133 is formed over the insulating film 130 (see FIG.28A). The above steps can be performed with reference to Embodiment 1.

Then, the opening 117 reaching the conductive film 113 is formed in theinsulating film 128, the insulating film 130, and the insulating film133 to form the insulating film 129, the insulating film 131, and theinsulating film 132 (see FIG. 28A). The pixel electrode 121 in contactwith the conductive film 113 through the opening 117 is formed (see FIG.26). The above steps can also be performed with reference to Embodiment1.

Through the above steps, the semiconductor device of this embodiment canbe fabricated.

As described above, the use of the semiconductor film formed in the sameformation step as the semiconductor film included in the transistor, forone electrode of the capacitor, allows fabrication of a semiconductordevice including the capacitor whose charge capacity is increased whileimproving the aperture ratio. As a result, the semiconductor device canhave an excellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in the oxidesemiconductor film, which is a semiconductor film included in thetransistor, are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments and example.

Embodiment 4

In this embodiment, a semiconductor device of one embodiment of thepresent invention will be described taking, as an example, a fringefield switching (FFS) mode liquid crystal display device in which liquidcrystal molecules are oriented with a lateral electric field. Note thatthe above embodiment can be referred to for components in thesemiconductor device described in this embodiment which are similar tothose of the semiconductor device described in the above embodiment.

<Structure of Semiconductor Device>

FIGS. 40A and 40B are top views of a pixel 501 described in thisembodiment. FIG. 40A is a top view of the pixel 501 where a commonelectrode 521 is not provided, and FIG. 40B is a top view of the pixel501 where the common electrode 521 is provided in FIG. 40A.

The pixel 501 in FIGS. 40A and 40B includes the transistor 103 and acapacitor 505 connected to the transistor 103. The capacitor 505includes a semiconductor film 519 having higher conductivity than thesemiconductor film 111, a common electrode 521 formed using alight-transmitting conductive film, and a light-transmitting insulatingfilm (not illustrated in FIGS. 40A and 40B) included in the transistor103. That is to say, the capacitor 505 has a light-transmittingproperty. Further, the semiconductor film 519 having higher conductivitythan the semiconductor film 111 is connected to the conductive film 113in the transistor 103 and functions as a pixel electrode. The commonelectrode 521 has openings (slits). By application of an electric fieldbetween the common electrode and the pixel electrode, a region where thesemiconductor film 519, the light-transmitting insulating film, and thecommon electrode 521 overlap one another functions as a capacitor andthe liquid crystals can be controlled so as to be oriented in thedirection parallel with a substrate. Thus, an FFS mode liquid crystaldisplay device achieves a wide viewing angle and high image quality.

FIG. 41 is a cross-sectional view of the substrate 102 alongdashed-dotted line A1-A2 in FIG. 40B.

A cross-sectional structure of the pixel 501 of this embodiment is asfollows. A scan line 107 including the gate electrode of the transistor103 is provided over the substrate 102. The gate insulating film 127 isprovided over the scan line 107. The semiconductor film 111 is providedover a portion of the gate insulating film 127 which overlaps with thescan line 107, and the semiconductor film 519 having higher conductivitythan the semiconductor film 111 is provided over the gate insulatingfilm 127. The signal line 109 including the source electrode of thetransistor 103 and the conductive film 113 including the drain electrodeof the transistor 103 are provided over the semiconductor film 111 andthe gate insulating film 127. The conductive film 113 including thedrain electrode is connected to the semiconductor film 519, and thesemiconductor film 519 having higher conductivity than the semiconductorfilm 111 functions as a pixel electrode. The insulating film 129, theinsulating film 131, and the insulating film 132 functioning asprotective insulating films of the transistor 103 are provided over thegate insulating film 127, the signal line 109, the semiconductor film111, the conductive film 113, and the semiconductor film 519. The commonelectrode 521 is provided over the insulating film 129, the insulatingfilm 131, and the insulating film 132. The common electrode 521 isprovided continuously without being separated between pixels in thepixel portion. Note that a base insulating film may be provided betweenthe substrate 102, and the scan line 107 and the gate insulating film127.

The semiconductor film 519 having higher conductivity than thesemiconductor film 111 can be formed of a semiconductor film similar tothe semiconductor film 119 described in Embodiment 2 and thesemiconductor film 319 described in Embodiment 3, as appropriate. Thecommon electrode 521 can be formed using a material similar to that ofthe pixel electrode 121 described in Embodiment 1.

One electrode of the capacitor 505 of this embodiment is formed using asemiconductor film having higher conductivity than the semiconductorfilm 111 and connected to the conductive film 113 of the transistor,whereby the conductive film 113 and the semiconductor film 519 can bedirectly connected to each other without forming an opening, and theplanarity of the transistor 103 and the capacitor 505 can be improved.Further, a capacitor line is not provided and the common electrode 521having a light-transmitting property is made to function as a capacitorline, so that the aperture ratio of the pixel 501 can be furtherincreased.

Embodiment 5

In this embodiment, transistors which can be used in the scan linedriver circuit 104 and the signal line driver circuit 106 will bedescribed with reference to FIG. 36B, FIG. 42, FIGS. 43A and 43B, andFIGS. 44A and 44B.

A transistor 685 illustrated in FIG. 36B includes a gate electrode 607over the substrate 102, the gate insulating film 127 over the gateelectrode 607, the semiconductor film 111 over a portion of the gateinsulating film 127 which overlaps with the gate electrode 607, and asource electrode 609 and the drain electrode 613 over the semiconductorfilm 111 and the gate insulating film 127. Further, the insulating film129, the insulating film 131, and the insulating film 132 serving asprotective insulating films of the transistor 685 are provided over thegate insulating film 127, the source electrode 609, the semiconductorfilm 111, and the drain electrode 613. The conductive film 687 isprovided over the insulating film 132. The conductive film 687 overlapswith the gate electrode 607 with the semiconductor film 111 interposedtherebetween.

In the transistor 685, the conductive film 687 overlapping the gateelectrode 607 with the semiconductor film 111 interposed therebetween isprovided, whereby a variation in gate voltage at which an on-currentrises at different drain voltages can be reduced. Further, a currentflowing between the source electrode and the drain electrode in a sideof the semiconductor film 111 facing the conductive film 687 can becontrolled and thus variations in electrical characteristics betweendifferent transistors can be reduced. In addition, the provision of theconductive film 687 leads to a reduction in effect of a change inambient electric field on the semiconductor film 111; therefore, thereliability of the transistor can be improved. Further, when thepotential of the conductive film 687 is the same or substantially thesame as the minimum potential (Vss; for example, the potential of thesource electrode 609 in the case where the potential of the sourceelectrode 609 is a reference potential), a variation in thresholdvoltage of the transistor can be reduced and the reliability of thetransistor can be improved.

Note that it is preferable that the length of the width of theconductive film 687 between the source electrode 609 and the drainelectrode 613 be smaller than the distance between the source electrode609 and the drain electrode 613. In other words, it is preferable thatthe conductive film 687 be provided in a position overlapping part of achannel formation region in the semiconductor film 111 of the transistor685. When the conductive film 687 is provided in such a manner and thedistance between the semiconductor film 111 and the conductive film 687is small, that is, the insulating film 129, the insulating film 131, andthe insulating film 132 serving as protective insulating films are thin,an effect of an electric field on the conductive film 687 can be reducedand the range of variation in threshold voltage of the transistor 685can be reduced.

Calculation results of voltages applied to the conductive film 687 inthe transistor 685 and operations of the transistor will be describedwith reference to FIG. 42, FIGS. 43A and 43B, and FIGS. 44A and 44B.

FIG. 42 illustrates the structure of a transistor used for thesimulation. Note that device simulation software “Atlas” produced bySilvaco Inc. was used for the calculation.

In the transistor in FIG. 42, a gate insulating film 703 is providedover a gate electrode 701; an oxide semiconductor film 705 is providedas a semiconductor film over the gate insulating film 703; a sourceelectrode 707 and a drain electrode 709 are provided over the oxidesemiconductor film 705; an insulating film 711 serving as a protectiveinsulating film is provided over the gate insulating film 703, the oxidesemiconductor film 705, the source electrode 707, and the drainelectrode 709; and a conductive film 713 is provided over the insulatingfilm 711.

Note that in the calculation, the work function φM of the gate gateelectrode 701 was set to 5.0 eV. The gate insulating film 703 had alayered structure of a 400-nm-thick film with a dielectric constant of7.5 and a 50-nm-thick film with a dielectric constant of 4.1. The oxidesemiconductor film 705 was a single IGZO (111) layer. The band gap Eg ofthe IGZO layer was 3.15 eV, the electron affinity χ was 4.6 eV, thedielectric constant was 15, the electron mobility was 10 cm²/Vs, and thedonor density Nd was 1×10¹³/cm³. The work function φsd of the sourceelectrode 707 and the drain electrode 709 was 4.6 eV and the ohmiccontact between the oxide semiconductor film 705 and the sourceelectrode 707 and the drain electrode 709 was obtained. The dielectricconstant of the insulating film 711 was 3.9 and the thickness thereofwas 550 nm. The work function φM of the conductive film 713 was 4.8 eV.Note that defect levels, surface scattering, and the like in the oxidesemiconductor film 705 were not considered. The channel length and thechannel width of the transistor were 3 μm and 50 μm, respectively.

Next, FIGS. 43A and 43B show calculation results of the Id-Vgcharacteristics of a transistor where the potential of the conductivefilm 713 is floating and a transistor where the potential of theconductive film 713 is fixed to 0 V.

FIG. 43A shows equipotential curves in the case where the gate electrode701 of the transistor is supplied with a potential of 0 V, the sourceelectrode 707 is supplied with a potential of 0 V, the drain electrode709 is supplied with a potential of 10 V, and the conductive film 713 isfloating. FIG. 43B shows equipotential curves in the case where the gateelectrode 701 of the transistor is supplied with a potential of 0 V, thesource electrode 707 is supplied with a potential of 0 V, the drainelectrode 709 is supplied with a potential of 10 V, and the conductivefilm 713 is supplied with the potential equal to that of the sourceelectrode 707, here, a potential of 0 V.

In FIGS. 43A and 43B, dashed arrows indicate the direction of anelectric field in the insulating film 711. The electric field isgenerated from the high potential side to the low potential side in thedirection perpendicular to the equipotential curves. FIGS. 44A and 44Bshow current-voltage curves of the transistors illustrated in FIGS. 43Aand 43B. The horizontal axis represents voltage of the gate electrodeand the longitudinal axis represents current of the drain electrode. InFIGS. 44A and 44B, curves obtained by plotting black dots is acurrent-voltage curve in the case where the drain voltage (Vd) is 1 V,and curves obtained by plotting white dots is a current-voltage curve inthe case where the drain voltage (Vd) is 10 V.

The current-voltage curves in FIG. 44A show that in the case where theconductive film 713 is floating, the gate voltage at which the on-statecurrent starts to flow is more on the negative side when the drainvoltage Vd is 10 V than when the drain voltage Vd is 1 V. That is tosay, the gate voltage at which the on-state current starts to flowdepends on the drain voltage.

When the gate voltage is 0 V and the drain voltage is 10 V, an electricfield from the conductive film 713 to a back channel of the oxidesemiconductor film 705 is generated as shown by the dashed arrows inFIG. 43A. The potential of the conductive film 713 is raised toapproximately 5 V because a drain voltage (Vd) of 10 V is applied.Further, the conductive film 713 is close to the oxide semiconductorfilm 705; thus, the potential of the conductive film 713 effectivelyserves as a positive potential. Therefore, electrons are excessivelyinduced to the back channel side and a current flowing through the backchannel increases and accordingly, the threshold voltage of thecurrent-voltage characteristics is shifted in the negative direction.

On the other hand, the gate voltage at which the on-state current startsto flow of one of the current-voltage curves in FIG. 44B corresponds tothat of the other regardless of the drain voltage.

As in FIG. 43B, in the insulating film 711, an electric field isgenerated from the drain electrode 709 to the conductive film 713, whichimplies that the conductive film 713 functions so that electrons on theback channel side are substantially excluded. Thus, the gate voltage atwhich the on-state current starts to flow is slightly more on thepositive side than that of the curves in FIG. 44A.

From the above description, when a conductive film is provide so as tooverlap a channel formation region of an oxide semiconductor film andthe potential of the conductive film is fixed to 0 V, variations in gatevoltage at which the on-state current starts to flow at different drainvoltages can be reduced.

Embodiment 6

In this embodiment, one embodiment which can be applied to an oxidesemiconductor film, which is a semiconductor film, in the transistor andthe capacitor included in the semiconductor device described in theabove embodiment will be described.

The oxide semiconductor film is preferably formed using any of anamorphous oxide semiconductor, a single crystal oxide semiconductor, apolycrystalline oxide semiconductor, and an oxide semiconductorincluding a crystalline portion (a c-axis aligned crystalline oxidesemiconductor (CAAC-OS).

The CAAC-OS film is one of oxide semiconductor films including aplurality of crystal parts, and most of the crystal parts each fitinside a cube whose one side is less than 100 nm. Thus, there is a casewhere a crystal part included in the CAAC-OS film fits inside a cubewhose one side is less than 10 nm, less than 5 nm, or less than 3 nm.The density of defect states of the CAAC-OS film is lower than that ofthe microcrystalline oxide semiconductor film. The CAAC-OS film will bedescribed in detail below.

In a transmission electron microscope (TEM) image of the CAAC-OS film, aboundary between crystal parts, that is, a grain boundary is not clearlyobserved. Thus, in the CAAC-OS film, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

According to the TEM image of the CAAC-OS film observed in a directionsubstantially parallel with a sample surface (cross-sectional TEMimage), metal atoms are arranged in a layered manner in the crystalparts. Each metal atom layer has a morphology reflected by a surfaceover which the CAAC-OS film is formed (hereinafter, a surface over whichthe CAAC-OS film is formed is referred to as a formation surface) or atop surface of the CAAC-OS film, and is arranged in parallel with theformation surface or the top surface of the CAAC-OS film.

On the other hand, according to the TEM image of the CAAC-OS filmobserved in a direction substantially perpendicular to the samplesurface (plan TEM image), metal atoms are arranged in a triangular orhexagonal configuration in the crystal parts. However, there is noregularity of arrangement of metal atoms between different crystalparts.

From the results of the cross-sectional TEM image and the plan TEMimage, alignment is found in the crystal parts in the CAAC-OS film.

A CAAC-OS film is subjected to structural analysis with an X-raydiffraction (XRD) apparatus. For example, when the CAAC-OS filmincluding an InGaZnO₄ crystal is analyzed by an out-of-plane method, apeak appears frequently when the diffraction angle (2θ) is around 31°.This peak is derived from the (009) plane of the InGaZnO₄ crystal, whichindicates that crystals in the CAAC-OS film have c-axis alignment, andthat the c-axes are aligned in a direction substantially perpendicularto the formation surface or the top surface of the CAAC-OS film.

On the other hand, when the CAAC-OS film is analyzed by an in-planemethod in which an X-ray enters a sample in a direction substantiallyperpendicular to the c-axis, a peak appears frequently when 20 is around56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal.Here, analysis (φ scan) is performed under conditions where the sampleis rotated around a normal vector of a sample surface as an axis (φaxis) with 2θ fixed at around 56°. In the case where the sample is asingle-crystal oxide semiconductor film of InGaZnO₄, six peaks appear.The six peaks are derived from crystal planes equivalent to the (110)plane. On the other hand, in the case of a CAAC-OS film, a peak is notclearly observed even when φ scan is performed with 2θ fixed at around56°.

According to the above results, in the CAAC-OS film having c-axisalignment, while the directions of a-axes and b-axes are differentbetween crystal parts, the c-axes are aligned in a direction parallelwith a normal vector of a formation surface or a normal vector of a topsurface. Thus, each metal atom layer arranged in a layered mannerobserved in the cross-sectional TEM image corresponds to a planeparallel with the a-b plane of the crystal.

Note that the crystal part is formed concurrently with deposition of theCAAC-OS film or is formed through crystallization treatment such as heattreatment. As described above, the c-axis of the crystal is aligned witha direction parallel with a normal vector of a formation surface or anormal vector of a top surface. Thus, for example, in the case where ashape of the CAAC-OS film is changed by etching or the like, the c-axismight not be necessarily parallel with a normal vector of a formationsurface or a normal vector of a top surface of the CAAC-OS film.

Further, the degree of crystallinity in the CAAC-OS film is notnecessarily uniform. For example, in the case where crystal growthleading to the CAAC-OS film occurs from the vicinity of the top surfaceof the film, the degree of the crystallinity in the vicinity of the topsurface is higher than that in the vicinity of the formation surface insome cases. Further, when an impurity is added to the CAAC-OS film, thecrystallinity in a region to which the impurity is added is changed, andthe degree of crystallinity in the CAAC-OS film varies depending onregions.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed byan out-of-plane method, a peak of 2θ may also be observed at around 36°,in addition to the peak of 2θ at around 31°. The peak of 2θ at around36° indicates that a crystal having no c-axis alignment is included inpart of the CAAC-OS film. It is preferable that in the CAAC-OS film, apeak of 2θ appear at around 31° and a peak of 2θ do not appear at around36°.

There are three methods for forming a CAAC-OS film.

The first method is to form an oxide semiconductor film at a temperaturein the range of 100° C. to 450° C. to form, in the oxide semiconductorfilm, crystal parts in which the c-axes are aligned in the directionparallel with a normal vector of a surface where the oxide semiconductorfilm is formed or a normal vector of a surface of the oxidesemiconductor film.

The second method is to form an oxide semiconductor film with a smallthickness and then heat it at a temperature in the range of 200° C. to700° C., to form, in the oxide semiconductor film, crystal parts inwhich the c-axes are aligned in the direction parallel with a normalvector of a surface where the oxide semiconductor film is formed or anormal vector of a surface of the oxide semiconductor film.

The third method is to form a first oxide semiconductor film with asmall thickness, then heat it at a temperature in the range of 200° C.to 700° C., and form a second oxide semiconductor film to form, in thesecond oxide semiconductor film, crystal parts in which the c-axes arealigned in the direction parallel with a normal vector of the surfacewhere the second oxide semiconductor film is formed or to a normalvector of the top surface of the second oxide semiconductor film.

In a transistor using the CAAC-OS film as the oxide semiconductor film,change in electrical characteristics due to irradiation with visiblelight or ultraviolet light is small. Thus, the transistor using theCAAC-OS film as the oxide semiconductor film has high reliability.

Further, it is preferable that the CAAC-OS film be formed by asputtering method using a polycrystalline oxide semiconductor sputteringtarget. When ions collide with the sputtering target, a crystal regionincluded in the sputtering target may be separated from the target alongan a-b plane; in other words, a flat-plate-like or pellet-like sputteredparticle having a plane parallel with an a-b plane may flake off fromthe sputtering target. In this case, the flat-plate-like or pellet-likesputtered particle reaches a surface where the CAAC-OS film is to bedeposited while maintaining its crystal state, whereby the CAAC-OS filmcan be deposited.

For the deposition of the CAAC-OS film, the following conditions arepreferably used.

By reducing the mixing of impurities during the deposition, the crystalstate can be prevented from being broken by the impurities. For example,the concentration of impurities (e.g., hydrogen, water, carbon dioxide,or nitrogen) which exist in the deposition chamber may be reduced.Furthermore, the concentration of impurities in a deposition gas may bereduced. Specifically, a deposition gas whose dew point is −80° C. orlower, preferably −100° C. or lower is used.

By increasing the heating temperature of the surface where the CAAC-OSfilm is formed (for example, the substrate heating temperature) duringthe deposition, migration of a sputtered particle is likely to occurafter the sputtered particle reaches the surface where the CAAC-OS filmis formed. Specifically, the temperature of the surface where theCAAC-OS film is formed during the deposition is higher than or equal to100° C. and lower than or equal to 740° C., preferably higher than orequal to 150° C. and lower than or equal to 500° C. By increasing thetemperature of the surface where the CAAC-OS film is formed during thedeposition, when the flat-plate-like or pellet-like sputtered particlereaches the surface where the CAAC-OS film is formed, migration occurson the surface, so that flat planes of the sputtered particles areattached to the surface.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is 30 vol % or higher, preferably 100 vol %.

As an example of the sputtering target, an In—Ga—Zn-based oxide targetis described below.

The polycrystalline In—Ga—Zn-based oxide target is made by mixingInO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predeterminedmolar ratio, applying pressure, and performing heat treatment at atemperature higher than or equal to 1000° C. and lower than or equal to1500° C. This pressure treatment may be performed while cooling isperformed or may be performed while heating is performed. X, Y, and Zare each a given positive number. Here, the predetermined molar ratio ofInO_(X) powder to GaO_(Y) powder and ZnO_(Z) powder is, for example,2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powders andthe molar ratio for mixing the powders may be determined as appropriatedepending on the desired sputtering target.

Further, the oxide semiconductor film may have a structure in which aplurality of oxide semiconductor films are stacked. For example, theoxide semiconductor film may have a layered structure of a first oxidesemiconductor film and a second oxide semiconductor film which areformed using metal oxides with different atomic ratios. For example, thefirst oxide semiconductor film may be formed using one of an oxidecontaining two kinds of metals, an oxide containing three kinds ofmetals, and an oxide containing four kinds of metals, and the secondoxide semiconductor film may be formed using one of the above which isdifferent from the one used for the first oxide semiconductor film.

Alternatively, the oxide semiconductor film may have a two-layerstructure where the constituent elements of the first oxidesemiconductor film and the second oxide semiconductor film are the samewhile the atomic ratios of the constituent elements of the first oxidesemiconductor film and the second oxide semiconductor film aredifferent. For example, the first oxide semiconductor film may containIn, Ga, and Zn at an atomic ratio of 3:1:2, and the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of1:1:1. Alternatively, the first oxide semiconductor film may contain In,Ga, and Zn at an atomic ratio of 2:1:3, and the second oxidesemiconductor film may contain In, Ga, and Zn at an atomic ratio of1:3:2. Note that a proportion of each atom in the atomic ratio of theoxide semiconductor film varies within a range of ±20% as an error.

In this case, in one of the first oxide semiconductor film and thesecond oxide semiconductor film, which is closer to the gate electrode(the oxide semiconductor film on the channel side), the atomic ratio ofIn to Ga is preferably as follows: In≥Ga. In the other oxidesemiconductor film, which is farther from the gate electrode (the oxidesemiconductor film on the back channel side), the atomic ratio of In toGa is preferably as follows: In<Ga. With a layered structure of theseoxide semiconductor films, a transistor having high field-effectmobility can be formed. On the other hand, the atomic ratio of In to Gain the oxide semiconductor film closer to the gate electrode (the oxidesemiconductor film on the channel side) satisfies the relation In<Ga andthe atomic ratio of In to Ga in the oxide semiconductor film on the backchannel side satisfies the relation In ≥Ga, whereby a variation inthreshold voltage of a transistor due to a change over time or areliability test can be reduced.

The first oxide semiconductor film containing In, Ga, and Zn at anatomic ratio of 1:3:2 can be formed by a sputtering method using anoxide target with an atomic ratio of 1:3:2 under the conditions wherethe substrate temperature is room temperature and a sputtering gas isargon or a mixed gas of argon and oxygen. The second oxide semiconductorfilm containing In, Ga, and Zn at an atomic ratio of 3:1:2 can be formedby a sputtering method using an oxide target with an atomic ratio of3:1:2 in a manner similar to that of the first oxide semiconductor film.

Alternatively, the oxide semiconductor film may have a three-layerstructure of a first oxide semiconductor film, a second oxidesemiconductor film, and a third oxide semiconductor film, in which theconstituent elements thereof are the same and the atomic ratios of theconstituent elements of the first oxide semiconductor film, the secondoxide semiconductor film, and the third oxide semiconductor film aredifferent. The case where the oxide semiconductor film has a three-layerstructure will be described with reference to FIG. 29.

In a transistor illustrated in FIG. 29, a first oxide semiconductor film199 a, a second oxide semiconductor film 199 b, and a third oxidesemiconductor film 199 c are stacked in this order from the gateinsulating film 127 side. As a material of the first oxide semiconductorfilm 199 a and the third oxide semiconductor film 199 c, a materialrepresented by InM1_(x)Zn_(y)O_(z) (x≥1, y>1, z>0, M1=Ga, Hf, or thelike) is used. Note that in the case where a material of the first oxidesemiconductor film 199 a and the third oxide semiconductor film 199 ccontains Ga, a material containing a large proportion of Ga,specifically, a material which can be represented by InM1_(x)Zn_(y)O_(z)where x is larger than 10 is unsuitable because powder might begenerated in deposition.

As a material of the second oxide semiconductor film 199 b, a materialwhich can be represented by InM2_(x)Zn_(y)O₂ (x≥1, y≥x, z>0, M2=Ga, Sn,or the like) is used.

Materials of the first to third oxide semiconductor films 199 a to 199 care appropriately selected so that a well structure is formed in whichthe conduction band of the second oxide semiconductor film 199 b isdeeper from the vacuum level than the conduction bands of the first andthird oxide semiconductor films 199 a and 199 c.

Note that silicon and carbon, which are Group 14 elements, are donorsupply sources in an oxide semiconductor film, so that silicon or carboncontained in an oxide semiconductor film makes it n-type. Thus, theconcentration of silicon contained in an oxide semiconductor film andthe concentration of carbon contained in an oxide semiconductor film areeach less than or equal to 3×10¹⁸/cm³, preferably less than or equal to3×10¹⁷/cm³. It is particularly preferable to employ a structure wherethe first and third oxide semiconductor films 199 a and 199 c sandwichor surround the second oxide semiconductor film 199 b serving as acarrier path so that a large number of Group 14 elements do not enterthe second oxide semiconductor film 199 b. That is to say, the first andthird oxide semiconductor films 199 a and 199 c can also be calledbarrier films which prevent Group 14 elements such as silicon and carbonfrom entering the second oxide semiconductor film 199 b.

For example, the atomic ratio of In to Ga and Zn in the first oxidesemiconductor film 199 a may be 1:3:2, the atomic ratio of In to Ga andZn in the second oxide semiconductor film 199 b may be 3:1:2, and theatomic ratio of In to Ga and Zn in the third oxide semiconductor film199 c may be 1:1:1. Note that the third oxide semiconductor film 199 ccan be formed by a sputtering method using an oxide target containingIn, Ga, and Zn at an atomic ratio of 1:1:1.

Alternatively, a three-later structure may be employed in which thefirst oxide semiconductor film 199 a contains In, Ga, and Zn at anatomic ratio of 1:3:2, the second oxide semiconductor film 199 bcontains In, Ga, and Zn at an atomic ratio of 1:1:1 or 1:3:2, and thethird oxide semiconductor film 199 c contains In, Ga, and Zn at anatomic ratio of 1:3:2.

Since the constituent elements of the first to third oxide semiconductorfilms 199 a to 199 c are the same, the second oxide semiconductor film199 b has fewer defect states (trap levels) at the interface with thefirst oxide semiconductor film 199 a. Specifically, the defect states(trap levels) are fewer than those at the interface between the gateinsulating film 127 and the first oxide semiconductor film 199 a. Forthis reason, when the oxide semiconductor films are stacked in the abovemanner, a variation in the threshold voltage of a transistor due to achange over time or a reliability test can be reduced.

Further, when materials of the first to third oxide semiconductor films199 a to 199 c are appropriately selected so that a well structure isformed in which the conduction band of the second oxide semiconductorfilm 199 b is deeper from the vacuum level than the conduction bands ofthe first and third oxide semiconductor films, the field-effect mobilityof the transistor can be increased and a variation in the thresholdvoltage of the transistor due to a change over time or a reliabilitytest can be reduced.

Further, the first to third oxide semiconductor films 199 a to 199 c maybe formed using oxide semiconductor films having differentcrystallinities. That is, the first to third oxide semiconductor filmsmay be formed using any of a single crystal oxide semiconductor film, apolycrystalline oxide semiconductor film, a microcrystalline oxidesemiconductor film, an amorphous oxide semiconductor film, and a CAAC-OSfilm, as appropriate. When an amorphous oxide semiconductor film is usedas any one of the first to third oxide semiconductor films 199 a to 199c, internal stress or external stress of the oxide semiconductor film isrelieved, variations in characteristics of a transistor is reduced and avariation in the threshold voltage of the transistor due to a changeover time or a reliability test can be reduced.

At least the second oxide semiconductor film 199 b, which can serve as achannel formation region, is preferably a CAAC-OS film. An oxidesemiconductor film on the back channel side, in this embodiment, thethird oxide semiconductor film 199 c is preferably an amorphous oxidesemiconductor film or a CAAC-OS film. With such a structure, a variationin the threshold voltage of a transistor due to a change over time or areliability test can be reduced.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments and example.

Embodiment 7

A semiconductor device (also referred to as a display device) having adisplay function can be fabricated using a transistor and a capacitorexamples of which are described in the above embodiments. Further, partor all of a driver circuit which includes a transistor can be formedover a substrate where a pixel portion is formed, whereby asystem-on-panel can be formed. In this embodiment, examples of displaydevices using the transistor examples which are shown in the aboveembodiments will be described with reference to FIGS. 30A to 30C, FIGS.31A and 31B, and FIGS. 32A to 32C. FIGS. 31A and 31B are cross-sectionalviews illustrating cross-sectional structures taken along dashed-dottedline M-N in FIG. 30B. Note that FIGS. 31A and 31B each illustrate onlypart of the structure of a pixel portion.

In FIG. 30A, a sealant 905 is provided so as to surround a pixel portion902 provided over a first substrate 901, and the pixel portion 902 issealed with the sealant 905 and a second substrate 906. In FIG. 30A, asignal line driver circuit 903 and a scan line driver circuit 904 eachare formed using a single-crystal semiconductor or a polycrystallinesemiconductor over a substrate prepared separately, and mounted in aregion different from the region surrounded by the sealant 905 over thefirst substrate 901. Further, various signals and potentials aresupplied to the signal line driver circuit 903, the scan line drivercircuit 904, and the pixel portion 902 from flexible printed circuits(FPCs) 918 a and 918 b.

In FIGS. 30B and 30C, the sealant 905 is provided so as to surround thepixel portion 902 and the scan line driver circuit 904 which areprovided over the first substrate 901. The second substrate 906 isprovided over the pixel portion 902 and the scan line driver circuit904. Thus, the pixel portion 902 and the scan line driver circuit 904are sealed together with a display element, with the first substrate901, the sealant 905, and the second substrate 906. In FIGS. 30B and30C, a signal line driver circuit 903 formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateseparately prepared is mounted in a region different from the regionsurrounded by the sealant 905 over the first substrate 901. In FIGS. 30Band 30C, various signals and potentials are supplied to the signal linedriver circuit 903, the scan line driver circuit 904, and the pixelportion 902 from an FPC 918.

Although FIGS. 30B and 30C each illustrate an example in which thesignal line driver circuit 903 is formed separately and mounted on thefirst substrate 901, this structure is not necessarily employed. Thescan line driver circuit may be separately formed and then mounted, oronly part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 30A illustrates an example in which the signal line drivercircuit 903 and the scan line driver circuit 904 are mounted by a COGmethod. FIG. 30B illustrates an example in which the signal line drivercircuit 903 is mounted by a COG method. FIG. 30C illustrates an examplein which the signal line driver circuit 903 is mounted by a TAB method.

The display device includes in its category a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel.

Note that the display device in this specification refers to an imagedisplay device or a display device. The display device may serve as alight source (including a lighting device). Furthermore, the displaydevice also includes all the following modules in its category: a moduleto which a connector such as an FPC or a TCP is attached; a modulehaving a TCP at the tip of which a printed wiring board is provided; anda module in which an integrated circuit (IC) is directly mounted on adisplay element by a COG method.

The pixel portion and the scan line driver circuit which are providedover the first substrate include a plurality of transistors; any of thetransistors described in the above embodiments can be used therein.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by current orvoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element and an organic EL element. Furthermore,a display medium whose contrast is changed by an electric effect ofelectronic ink or the like can be used. FIGS. 31A and 31B eachillustrates an example of a liquid crystal display device including aliquid crystal element as a display element.

The liquid crystal display device illustrated in FIG. 31A is a verticalelectric field mode liquid crystal display device. The liquid crystaldisplay device includes a connection terminal electrode 915 and aterminal electrode 916. The connection terminal electrode 915 and theterminal electrode 916 are electrically connected to a terminal includedin the FPC 918 through an anisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930. The terminal electrode 916 isformed using the same conductive film as source and drain electrodes oftransistors 910 and 911.

Further, the pixel portion 902 and the scan line driver circuit 904which are provided over the first substrate 901 each include a pluralityof transistors, and the transistor 910 included in the pixel portion 902and the transistor 911 included in the scan line driver circuit 904 areillustrated as an examples. An insulating film 924 corresponding to theinsulating film 129, the insulating film 131, and the insulating film132 in Embodiment 1 is provided over the transistor 910 and thetransistor 911. Note that an insulating film 923 serves as a base film.

In this embodiment, the transistor described in Embodiment 1 can be usedas the transistor 910. Further, the transistor described in Embodiment 5in which the conductive film 917 is provided in a position overlappingpart of the channel formation region in the oxide semiconductor film ofthe transistor 911 can be used as the transistor 911. A capacitor 926 isformed using an oxide semiconductor film 927, the insulating film 924,and the first electrode 930. The oxide semiconductor film 927 isconnected to a capacitor line 929 through an electrode 928. Theelectrode 928 is formed using the same materials and steps as the sourceand drain electrodes of the transistors 910 and 911. The capacitor line929 is formed using the same materials and steps as gate electrodes ofthe transistors 910 and 911. Although the capacitor described inEmbodiment 1 is illustrated as the capacitor 926 here, any of thecapacitors in the other embodiments may be used as appropriate.

The transistor 910 included in the pixel portion 902 is electricallyconnected to a display element so that a display panel is formed. Thereis no particular limitation on the display element as long as displaycan be performed, and any of various kinds of display elements can beused.

A liquid crystal element 913 serving as a display element includes thefirst electrode 930, a second electrode 931, and a liquid crystal layer908. An insulating film 932 and an insulating film 933 each serving asan alignment film are provided so that the liquid crystal layer 908 isinterposed therebetween. The second electrode 931 is provided on thesecond substrate 906 side, and the first electrode 930 overlaps with thesecond electrode 931 with the liquid crystal layer 908 interposedtherebetween.

The first electrode and the second electrode (each of which is alsoreferred to as a pixel electrode, a common electrode, a counterelectrode, or the like) for applying voltage to the display element mayhave light-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 930 and the second electrode 931 can be formed usingmaterials similar to those of the pixel electrode 121 and the counterelectrode 154 in Embodiment 1 as appropriate.

A spacer 935 is a columnar spacer obtained by selectively etching aninsulating film and is provided in order to control the distance (cellgap) between the first electrode 930 and the second electrode 931.Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, thermotropic liquid crystal, low-molecular liquid crystal,high-molecular liquid crystal, polymer dispersed liquid crystal,ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or thelike can be used. Such a liquid crystal material exhibits a cholestericphase, a smectic phase, a cubic phase, a chiral nematic phase, anisotropic phase, or the like depending on a condition.

Alternatively, liquid crystal which exhibits a blue phase and for whichan alignment film is unnecessary may be used. A blue phase is one ofliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. The blue phase appears only in a narrowtemperature range; therefore, a liquid crystal composition into which achiral material is mixed in order to widen the temperature range is usedfor the liquid crystal layer. Note that the alignment film is formedusing an organic resin containing hydrogen, water, or the like, whichmight degrade the electrical characteristics of the transistor in thesemiconductor device of one embodiment of the present invention. In viewof the above, the use of liquid crystal which exhibits a blue phase forthe liquid crystal layer 160 enables fabrication of the semiconductordevice of one embodiment of the present invention without an organicresin, so that the semiconductor device can be highly reliable.

The first substrate 901 and the second substrate 906 are fixed in placeby the sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used. The sealant 925is in contact with the insulating film 924. The sealant 925 correspondsto the sealant 905 illustrated in FIGS. 30A to 30C.

In the liquid crystal display device, a black matrix (light-blockingfilm), an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member, and the likeare provided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anonlinear element.

Next, a transverse electric field mode liquid crystal display devicewill be described with reference to FIG. 31B. FIG. 31A is an FFS modeliquid crystal display device which is one example of transverseelectric field mode liquid crystal display devices. A structuredifferent from that of the transverse electric field mode liquid crystaldisplay device described in Embodiment 4 will be described.

In the liquid crystal display device illustrated in FIG. 31B, theconnection terminal electrode 915 is formed using the same material andsteps as a first electrode 940, and the terminal electrode 916 is formedusing the same material and steps as the source and drain electrodes ofthe transistors 910 and 911.

A liquid crystal element 943 includes the first electrode 940, a secondelectrode 941, and the liquid crystal layer 908 which are formed overthe insulating film 924. The first electrode 940 can be formed using thematerial of the first electrode 930 illustrated in FIG. 31A asappropriate. The planar shape of the first electrode 940 is a comb-likeshape, a staircase-like shape, a ladder-like shape, or the like. Thesecond electrode 941 functions as a common electrode and can be formedin a manner similar to that of the semiconductor film 119 described inEmbodiment 1. The insulating film 924 is provided between the firstelectrode 940 and the second electrode 941.

The second electrode 941 is connected to a common wiring 946 through anelectrode 945. Note that the electrode 945 is formed using the sameconductive film as the source and drain electrodes of the transistors910 and 911. The common wiring 946 is formed using the same material andsteps as the gate electrodes of the transistors 910 and 911. Althoughthe description is made using the capacitor described in Embodiment 1 asthe liquid crystal element 943 here, any of the capacitors described inthe other embodiments can be used as appropriate.

FIGS. 32A to 32C illustrate examples of the liquid crystal displaydevice in FIG. 31A in which a common connection portion (pad portion)for being electrically connected to the second electrode 931 providedover the substrate 906 is formed over the substrate 901.

The common connection portion is provided in a position overlapping thesealant 925 for bonding the substrate 901 and the substrate 906 and iselectrically connected to the second electrode 931 through conductiveparticles contained in the sealant 925. Alternatively, the commonconnection portion is provided in a position which does not overlap thesealant (except for the pixel portion) and a paste containing conductiveparticles is provided separately from the sealant 925 so as to overlapthe common connection portion, whereby the common connection portion iselectrically connected to the second electrode 931.

FIG. 32A is a cross-sectional view of the common connection portiontaken along I-J in the top view in FIG. 32B.

A common potential line 975 is provided over a gate insulating film 922and is formed using the same material and steps as source and drainelectrodes 971 and 973 of the transistor 910 illustrated in FIGS. 32Aand 32C.

Further, the common potential line 975 is covered with the insulatingfilm 924, and a plurality of openings are formed in the insulating film924 at positions overlapping the common potential line 975. Theseopenings are formed through the same steps as a contact hole whichconnects the first electrode 930 and one of the source electrode 971 andthe drain electrode 973 of the transistor 910.

Further, the common potential line 975 is connected to the commonelectrode 977 through the openings. The common electrode 977 is providedover the insulating film 924 and formed using the same material andsteps as the connection terminal electrode 915 and the first electrode930 in the pixel portion.

In this manner, the common connection portion can be formed in the sameprocess as the switching element in the pixel portion 902.

The common electrode 977 is in contact with the conductive particlescontained in the sealant and is electrically connected to the secondelectrode 931 of the substrate 906.

Alternatively, as illustrated in FIG. 32C, a common potential line 985may be formed using the same material and steps as the gate electrode ofthe transistor 910.

In the common connection portion in FIG. 32C, the common potential line985 is provided under the gate insulating film 922 and the insulatingfilm 924, and a plurality of openings are formed in the gate insulatingfilm 922 and the insulating film 924 at positions overlapping the commonpotential line 985. These openings are formed by etching the insulatingfilm 924 and further selectively etching the gate insulating film 922,through the same steps as a contact hole which connects the firstelectrode 930 and one of the source electrode 971 and the drainelectrode 973 of the transistor 910.

Further, the common potential line 985 is connected to the commonelectrode 987 through the openings. The common electrode 987 is providedover the insulating film 924 and formed using the same material andsteps as the connection terminal electrode 915 and the first electrode930 in the pixel portion.

As described above, the use of the transistor and capacitor described inthe above embodiment allows fabrication of a semiconductor deviceincluding the capacitor whose charge capacity is increased whileimproving the aperture ratio. As a result, the semiconductor device canhave an excellent display quality.

Further, oxygen vacancies and impurities such as hydrogen in the oxidesemiconductor film, which is a semiconductor film included in thetransistor, are reduced, so that the semiconductor device of oneembodiment of the present invention has favorable electricalcharacteristics.

Note that the structures and the like described in this embodiment canbe combined as appropriate with any of the structures and the likedescribed in the other embodiments and example.

Embodiment 8

The semiconductor device of one embodiment of the present invention canbe used in various electronic devices (including game machines).Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, cameras such as a digital camera and a digital video camera, adigital photo frame, a mobile phone, a portable game machine, a portableinformation terminal, an audio reproducing device, game machines (e.g.,a pachinko machine and a slot machine), and a game console. FIGS. 33A to33C illustrate examples of these electronic devices.

FIG. 33A illustrates a table 9000 having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

Any of the semiconductor devices described in the above embodiments canbe used for the display portion 9003. Thus, the display portion 9003 canhave high display quality.

The display portion 9003 functions as a touch panel. When a user touchesdisplayed buttons 9004 which are displayed on the display portion 9003of the table 9000 with his/her finger or the like, the user can carryout operation on the screen and data input. Further, when the table maybe made to communicate with home appliances or control the homeappliances, the table 9000 may function as a control device whichcontrols the home appliances by operation on the screen. For example,with the use of a semiconductor device having an image sensor function,the display portion 9003 can function as a touch panel.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television set. When atelevision set having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 33B illustrates a television set 9100. In the television set 9100,a display portion 9103 is incorporated in a housing 9101 and an imagecan be displayed on the display portion 9103. Note that the housing 9101is supported by a stand 9105 here.

The television set 9100 can be operated with an operation switch of thehousing 9101 or a separate remote controller 9110. Channels and volumecan be controlled with an operation key 9109 of the remote controller9110 so that an image displayed on the display portion 9103 can becontrolled. Furthermore, the remote controller 9110 may be provided witha display portion 9107 for displaying data output from the remotecontroller 9110.

The television set 9100 illustrated in FIG. 33B is provided with areceiver, a modem, and the like. With the use of the receiver, thetelevision set 9100 can receive general television broadcasts. Moreover,when the television set 9100 is connected to a wired or wirelesscommunication network via the modem, one-way (from a sender to areceiver) or two-way (between a sender and a receiver or betweenreceivers) data communication can be performed.

Any of the semiconductor devices described in the above embodiments canbe used for the display portions 9103 and 9107. Thus, the television setcan have high display quality.

FIG. 33C illustrates a computer 9200 including a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, and a pointing device 9206.

Any of the semiconductor devices described in the above embodiments canbe used for the display portion 9203. Thus, the computer 9200 can havehigh display quality.

FIGS. 34A and 34B illustrate a foldable tablet terminal. FIG. 34Aillustrates the tablet terminal in the state of being unfolded. Thetablet terminal includes a housing 9630, a display portion 9631 a, adisplay portion 9631 b, a display-mode switching button 9034, a powerbutton 9035, a power-saving-mode switching button 9036, a fastener 9033,and an operation button 9038.

Any of the semiconductor devices described in the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b,so that the tablet terminal can have high reliability.

A touch panel area 9632 a can be provided in part of the display portion9631 a, in which area, data can be input by touching displayed operationkeys 9638. Note that half of the display portion 9631 a has only adisplay function and the other half has a touch panel function. However,the structure of the display portion 9631 a is not limited to this, andall the area of the display portion 9631 a may have a touch panelfunction. For example, a keyboard can be displayed on the whole displayportion 9631 a to be used as a touch panel, and the display portion 9631b can be used as a display screen.

A touch panel area 9632 b can be provided in part of the display portion9631 b like in the display portion 9631 a. When a keyboard displayswitching button 9639 displayed on the touch panel is touched with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 9631 b.

The touch panel area 9632 a and the touch panel area 9632 b can becontrolled by touch input at the same time.

The display-mode switching button 9034 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power-saving-mode switching button 9036allows optimizing the display luminance in accordance with the amount ofexternal light in use which is detected by an optical sensorincorporated in the tablet terminal. In addition to the optical sensor,other detecting devices such as sensors for determining inclination,such as a gyroscope or an acceleration sensor, may be incorporated inthe tablet terminal.

Although the display area of the display portion 9631 a is the same asthat of the display portion 9631 b in FIG. 34A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 9631 a may be different from that of the displayportion 9631 b, and further, the display quality of the display portion9631 a may be different from that of the display portion 9631 b. Forexample, one of the display portions 9631 a and 9631 b may displayhigher definition images than the other.

FIG. 34B illustrates the tablet terminal in the state of being closed.The tablet terminal includes the housing 9630, a solar cell 9633, and acharge and discharge control circuit 9634. FIG. 34B illustrates anexample where the charge and discharge control circuit 9634 includes abattery 9635 and a DC-DC converter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen the tablet terminal is not in use. Thus, the display portions 9631a and 9631 b can be protected, which permits the tablet terminal to havehigh durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 34A and 34B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 9633, which is attached on a surface of the tabletterminal, can supply electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630 and thus thebattery 9635 can be charged efficiently. The use of a lithium-ionbattery as the battery 9635 has advantages such as a reduction in size.

The structure and operation of the charge and discharge control circuitillustrated in FIG. 34B will be described with reference to a blockdiagram of FIG. 34C. FIG. 34C illustrates the solar cell 9633, thebattery 9635, the DC-DC converter 9636, a converter 9637, switches SW1to SW3, and the display portion 9631. The battery 9635, the DC-DCconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 34B.

First, an example of operation in the case where electric power isgenerated by the solar cell 9633 using external light will be described.The voltage of electric power generated by the solar cell is raised orlowered by the DC-DC converter 9636 so that the electric power has avoltage for charging the battery 9635. When the display portion 9631 isoperated with the electric power from the solar cell 9633, the switchSW1 is turned on and the voltage of the electric power is raised orlowered by the converter 9637 to a voltage needed for operating thedisplay portion 9631. In addition, when display on the display portion9631 is not performed, the switch SW1 is turned off and the switch SW2is turned on so that the battery 9635 may be charged.

Although the solar cell 9633 is described as an example of a powergeneration means, there is no particular limitation on the powergeneration means, and the battery 9635 may be charged with any of theother means such as a piezoelectric element or a thermoelectricconversion element (Peltier element). For example, the battery 9635 maybe charged with a non-contact power transmission module capable ofperforming charging by transmitting and receiving electric powerwirelessly (without contact), or any of the other charge means used incombination.

The structures and the like described in this embodiment can be combinedas appropriate with any of the structures and the like described in theother embodiments and example.

Example 1

In this example, a liquid crystal display device is fabricated usingEmbodiment 2. The specifications and display image of the liquid crystaldisplay device will be described.

In this example, a liquid crystal display device in which the gateinsulating film 227 had a two-layer structure and a semiconductor film119 in a capacitor 245 was in contact with the insulating film 225formed of a nitride insulating film so that the semiconductor film 119serving as one electrode of the capacitor was n-type was fabricated asillustrated in FIG. 24. Table 1 shows the specifications of the liquidcrystal display device, a signal line driver circuit, and a scan linedriver circuit.

TABLE 1 Liquid crystal display device Panel size 3.4 inch (portrait)Effective pixels 540 (H) × RGB × 960 (V) (qHD) Pixel size 0.026 mm (H) ×0.078 mm (V) External size 52.2 mm (H) × 93.1 mm (V) Display area 41.15mm (H) × 74.88 mm (V) Resolution 326 ppi Display element LCD (TN mode)Color method CF method Aperture ratio 60.00% Drive frequency 60 Hz Videosignal mode Analog dot-sequential Gate Driver Embedded Source DriverEmbedded VCOM ≤15 V Signal line driver cirucit Video signal voltage −5/5V Clock frequency 289.18 kHz Sampling period 432 ns Signal voltage−10/16 V Video division 45 pixels simultaneous sampling Scan line drivercircuit Clock frequency 14.46 kHz Signal voltage −14/14 V

Note that transistors provided in the signal line driver circuit and thescan line driver circuit each have a structure where a conductive filmis not provided over a protective insulating film as in a pixel portion.

Next, FIG. 45 shows a photograph of an image displayed by the liquidcrystal display device fabricated in this example. As shown in FIG. 45,the liquid crystal display device fabricated in this example can displaya high-quality image.

EXPLANATION OF REFERENCE

100: pixel portion, 101: pixel, 102: substrate, 103: transistor, 104:scan line driver circuit, 105: capacitor, 106: signal line drivercircuit, 107: scan line, 107 a: gate electrode, 108: liquid crystalelement, 109: signal line, 109 a: source electrode, 111: semiconductorfilm, 113: conductive film, 113 a: drain electrode, 115: capacitor line,117: opening, 119: semiconductor film, 121: pixel electrode, 123:opening, 125: conductive film, 126: insulating film, 127: gateinsulating film, 128: insulating film, 129: insulating film, 130:insulating film, 131: insulating film, 132: insulating film, 133:insulating film, 134: organic insulating film, 141: pixel, 143: opening,145: capacitor, 146: capacitor, 150: substrate, 151: pixel, 152:light-blocking film, 154: counter electrode, 156: insulating film, 158:insulating film, 160: liquid crystal layer, 161: pixel, 165: capacitor,167: conductive film, 169: transistor, 171: pixel, 172: pixel, 173:capacitor, 174: capacitor, 175: capacitor line, 176: capacitor line,177: semiconductor film, 178: semiconductor film, 182: channelprotective film, 183: transistor, 185: transistor, 187: conductive film,190: transistor, 191: signal line, 193: conductive film, 195:semiconductor film, 196: pixel, 197: capacitor, 198: semiconductor film,199: conductive film, 199 a: oxide semiconductor film, 199 b: oxidesemiconductor film, 199 c: oxide semiconductor film, 201: pixel, 205:capacitor, 221: pixel electrode, 225: insulating film, 226: insulatingfilm, 227: gate insulating film, 228: insulating film, 229: insulatingfilm, 230: insulating film, 231: insulating film, 232: insulating film,233: insulating film, 245: capacitor, 255: capacitor, 271: pixelelectrode, 279: insulating film, 281: insulating film, 282: insulatingfilm, 301: pixel, 305: capacitor, 307: gate electrode, 309: sourceelectrode, 315: capacitor line, 319: semiconductor film, 401_1: pixel,401_2: pixel, 403_1: transistor, 403_2: transistor, 405_1: capacitor,405_2: capacitor, 407_1: scan line, 407_2: scan line, 409: signal line,411_1: semiconductor film, 411_2: semiconductor film, 413_1: conductivefilm, 413_2: conductive film, 415: capacitor line, 417_1: opening,417_2: opening, 419_1: semiconductor film, 419_2: semiconductor film,421_1: pixel electrode, 421_2: pixel electrode, 423: opening, 425:conductive film, 431_1: pixel, 431_2: pixel, 433_1: transistor, 433_2:transistor, 435_1: capacitor, 435_2: capacitor, 437: scan line, 439_1:signal line, 439_2: signal line, 441_1: semiconductor film, 441_2:semiconductor film, 443_1: conductive film, 443_2: conductive film, 445:capacitor line, 447_1: opening, 447_2: opening, 449_1: semiconductorfilm, 449_2: semiconductor film, 451_1: pixel electrode, 451_2: pixelelectrode, 501: pixel, 505: capacitor, 519: semiconductor film, 521:common electrode, 607: gate electrode, 609: source electrode, 613: drainelectrode, 685: transistor, 687: conductive film, 701: gate electrode,703: gate insulating film, 705: oxide semiconductor film, 707: sourceelectrode, 709: drain electrode, 711: insulating film, 713: conductivefilm, 901: substrate, 902: pixel portion, 903: signal line drivercircuit, 904: scan line driver circuit, 905: sealant, 906: substrate,908: liquid crystal layer, 910: transistor, 911: transistor, 913: liquidcrystal element, 915: connection terminal electrode, 916: terminalelectrode, 917: conductive film, 918: FPC, 918 b: FPC, 919: anisotropicconductive agent, 922: gate insulating film, 923: insulating film, 924:insulating film, 925: sealant, 926: capacitor, 927: oxide semiconductorfilm, 928: electrode, 929: capacitor line, 930: electrode, 931:electrode, 932: insulating film, 933: insulating film, 935: spacer, 940:electrode, 941: electrode, 943: liquid crystal element, 945: electrode,946: common wiring, 971: source electrode, 973: drain electrode, 975:common potential line, 977: common electrode, 985: common potentialline, 987: common electrode, 9000: table, 9001: housing, 9002: legportion, 9003: display portion, 9004: displayed button, 9005: powercord, 9033: fastener, 9034: display-mode switching button, 9035: powerbutton, 9036: power-saving-mode switching button, 9038: operationbutton, 9100: television set, 9101: housing, 9103: display portion,9105: stand, 9107: display portion, 9109: operation key, 9110: remotecontroller, 9200: computer, 9201: main body, 9202: housing, 9203:display portion, 9204: keyboard, 9205: external connection port, 9206:pointing device, 9630: housing, 9631: display portion, 9631 a: displayportion, 9631 b: display portion, 9632 a: touch panel area, 9632 b:touch panel area, 9633: solar cell, 9634: charge and discharge controlcircuit, 9635: battery, 9636: DC-DC converter, 9637: converter, 9638:operation key, and 9639: button.

This application is based on Japanese Patent Application serial no.2012-173349 filed with the Japan Patent Office on Aug. 3, 2012, JapanesePatent Application serial no. 2012-filed with the Japan Patent Office onAug. 10, 2012, and Japanese Patent Application serial no. 2012-188093filed with the Japan Patent Office on Aug. 28, 2012, the entire contentsof which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: asubstrate; a first insulating film over the substrate; a secondinsulating film in direct contact with the first insulating film; athird insulating film in direct contact with the second insulating film;a gate electrode over the substrate; a semiconductor film over thesubstrate, overlapping with the gate electrode, including a channelformation region, sandwiched between the first insulating film and thesecond insulating film, and in direct contact with one of the firstinsulating film and the second insulating film; a first conductive filmand a second conductive film in electrical contact with thesemiconductor film; a pixel electrode in electrical contact with thefirst conductive film in an opening formed in the second insulating filmand the third insulating film; a transistor comprising: the gateelectrode; the semiconductor film; and the first insulating film betweenthe gate electrode and the semiconductor film; and a capacitorcomprising: a first capacitor electrode; the pixel electrode as a secondcapacitor electrode; and the third insulating film as a capacitordielectric film between the first capacitor electrode and the pixelelectrode, wherein the first capacitor electrode and the semiconductorfilm are formed from a same film, and wherein the first capacitorelectrode is in direct contact with the third insulating film and withthe one of the first insulating film and the second insulating film. 2.The semiconductor device according to claim 1, wherein the secondinsulating film is in direct contact with a periphery of the firstcapacitor electrode.
 3. The semiconductor device according to claim 1,wherein the first capacitor electrode further contains a dopant at aconcentration greater than 1×10¹⁹ atoms/cm³ and less than or equal to1×10²² atoms/cm³.
 4. The semiconductor device according to claim 1,wherein the first capacitor electrode contains a dopant so as to have ahigher electrical conductivity than the semiconductor film.
 5. Thesemiconductor device according to claim 1, further comprising an organicinsulating film interposed between the third insulating film and thepixel electrode, wherein the pixel electrode is in direct contact withthe third insulating film through an opening in the organic insulatingfilm.
 6. The semiconductor device according to claim 1, furthercomprising a capacitor line formed from a same film as the gateelectrode, wherein the first capacitor electrode is connected to thecapacitor line through a film formed from a same film as a sourceelectrode or a drain electrode of the transistor.
 7. The semiconductordevice according to claim 1, wherein the pixel electrode is in directcontact with the third insulating film in a region overlapping with thefirst capacitor electrode and the second capacitor electrode.
 8. Thesemiconductor device according to claim 1, further comprising a flexibleprinted circuit attached to the substrate and connected to the pixelelectrode, the semiconductor device being a display device.
 9. Asemiconductor device comprising: a substrate; a first insulating filmover the substrate, the first insulating film being a first oxide film;a second insulating film on and in direct contact with the firstinsulating film, the second insulating film being a second oxide film; athird insulating film on and in direct contact with the secondinsulating film, the third insulating film being a nitride film; a gateelectrode over the substrate; an oxide semiconductor film over thesubstrate, overlapping with the gate electrode, including a channelformation region, sandwiched between the first insulating film and thesecond insulating film, and in direct contact with the first insulatingfilm; a source electrode and a drain electrode in electrical contactwith the oxide semiconductor film; a light-transmitting pixel electrodein electrical contact one of the source electrode and the drainelectrode in an opening formed in the second insulating film and thethird insulating film; a transistor comprising: the gate electrode; theoxide semiconductor film; and the first insulating film between the gateelectrode and the oxide semiconductor film; and a capacitor comprising:a first capacitor electrode; the light-transmitting pixel electrode as asecond capacitor electrode; and the third insulating film as a capacitordielectric film between the first capacitor electrode and thelight-transmitting pixel electrode, wherein the first capacitorelectrode and the oxide semiconductor film are formed from a same film,and wherein the first capacitor electrode is in direct contact with thefirst insulating film and with the third insulating film.
 10. Thesemiconductor device according to claim 9, wherein the first capacitorelectrode further contains a dopant at a concentration greater than1×10¹⁹ atoms/cm³ and less than or equal to 1×10²² atoms/cm³.
 11. Thesemiconductor device according to claim 9, wherein the first capacitorelectrode contains a dopant so as to have a higher electricalconductivity than the oxide semiconductor film.
 12. The semiconductordevice according to claim 9, further comprising an organic insulatingfilm interposed between the third insulating film and thelight-transmitting pixel electrode, wherein the light-transmitting pixelelectrode is in direct contact with the third insulating film through anopening in the organic insulating film.
 13. The semiconductor deviceaccording to claim 9, further comprising a capacitor line formed from asame film as the gate electrode, wherein the first capacitor electrodeis connected to the capacitor line through a film formed from a samefilm as the source electrode or the drain electrode of the transistor.14. The semiconductor device according to claim 9, wherein thelight-transmitting pixel electrode is in direct contact with the thirdinsulating film in a region overlapping with the first capacitorelectrode and the second capacitor electrode.
 15. The semiconductordevice according to claim 9, further comprising a flexible printedcircuit attached to the substrate and connected to thelight-transmitting pixel electrode, the semiconductor device being adisplay device.
 16. A semiconductor device comprising: a substrate; afirst insulating film over the substrate, the first insulating filmbeing a first oxide film; a second insulating film on and in directcontact with the first insulating film, the second insulating film beinga second oxide film; a third insulating film on and in direct contactwith the second insulating film, the third insulating film being anitride film; a gate electrode over the substrate; an oxidesemiconductor film over the substrate, overlapping with the gateelectrode, including a channel formation region, sandwiched between thefirst insulating film and the second insulating film, and in directcontact with the first insulating film; a source electrode and a drainelectrode in electrical contact with the oxide semiconductor film; alight-transmitting pixel electrode in electrical contact one of thesource electrode and the drain electrode in an opening formed in thesecond insulating film and the third insulating film; a transistorcomprising: the gate electrode; the oxide semiconductor film; and thefirst insulating film between the gate electrode and the oxidesemiconductor film; and a capacitor comprising: a first capacitorelectrode; the light-transmitting pixel electrode as a second capacitorelectrode; and the third insulating film as a capacitor dielectric filmbetween the first capacitor electrode and the light-transmitting pixelelectrode, wherein the first capacitor electrode and the oxidesemiconductor film are formed from a same film, wherein the firstcapacitor electrode is in direct contact with the first insulating filmand with the third insulating film, and wherein the second insulatingfilm is in direct contact with a periphery of the first capacitorelectrode.
 17. The semiconductor device according to claim 16, whereinthe first capacitor electrode further contains a dopant at aconcentration greater than 1×10¹⁹ atoms/cm³ and less than or equal to1×10²² atoms/cm³.
 18. The semiconductor device according to claim 16,wherein the first capacitor electrode contains a dopant so as to have ahigher electrical conductivity than the oxide semiconductor film. 19.The semiconductor device according to claim 16, further comprising anorganic insulating film interposed between the third insulating film andthe light-transmitting pixel electrode, wherein the light-transmittingpixel electrode is in direct contact with the third insulating filmthrough an opening in the organic insulating film.
 20. The semiconductordevice according to claim 16, further comprising a capacitor line formedfrom a same film as the gate electrode, wherein the first capacitorelectrode is connected to the capacitor line through a film formed froma same film as the source electrode or the drain electrode of thetransistor.
 21. The semiconductor device according to claim 16, whereinthe light-transmitting pixel electrode is in direct contact with thethird insulating film in a region overlapping with the first capacitorelectrode and the second capacitor electrode.
 22. The semiconductordevice according to claim 16, further comprising a flexible printedcircuit attached to the substrate and connected to thelight-transmitting pixel electrode, the semiconductor device being adisplay device.